Fortification Friday: “Sufficient strength to resist” artillery taken on cavalry raids

Last week, we discussed Mahan’s American blockhouse and how that form of fortification became important during the Civil War.  During the war, the blockhouse became a common feature along railroads, roads, rivers, and other key points in the rear areas.  I don’t think this reflected a “brand new” use of the blockhouse fortifications, but rather one of greater significance as result of the needs of the war.

Pre-war thinking on such matters focused on a conflict against European powers, in a “War of 1812” scenario.  As such, the rear areas would be somewhat secure behind the Atlantic Ocean (with Mexico and the British in Canada assessed as more defensively oriented).  Only on the frontier would there be great need for blockhouses to secure supply and communication lines.  But the American Civil War upset that line of thinking.  With extended lines across half the continent, the armies could not expect to guard every quarter. This gave an opening for leaders with names like Stuart, Forrest, Wheeler, Mosby, and Morgan.  Yes, those glorious raiders riding about disrupting Yankee operations…

I would offer the Confederate raider threat reached its peak during the Atlanta Campaign.  Not to downplay other sectors, but the spring-summer of 1864 was somewhat a “point of no return” in many regards.  As a counter to the raiders, Major-General William T. Sherman directed the fortification of numerous posts along his supply and communication lines.  One of those we saw last week:

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And as I pointed out last week, the blockhouse in the photo compares well to the figures offered by Mahan in his post-war manual:

MahanPage65Fig1_2

Assuming Mahan’s figures are indeed close matches, we can project all sorts of details not visible in the photo – such as internal arrangements.  And thinking of those, we have the other half of Mahan’s illustrations to consider:

MahanPage65Fig4_5_6_7_8

Now we might say a picture is worth a thousand words.  If so, I’d offer these detailed figures are worth a couple thousand more. Figures 4, 5, and 6 give us a measure of the loop-holes on the lower story.  Figures 7 and 8 provide the same for the upper story.  One might dismiss the details as simply “common sense.”  But my counter would be that “common sense” is usually derived from experience.  And in this case, the manual attempts to impart some experience onto inexperienced cadets… who definitely needed sage advice based on wartime experience.  Besides, as I like to say on such matters, “it goes to show us how THEY did it.”

The caption provided for this page of figures further solidifies the linkage to wartime experiences:

Figs 1, 2, 3, etc., represent the chief details of the two-story block-houses that have been adopted for the defense of railroad stations, bridges, etc., along the line of communications of General Sherman’s Army.  From experiments made upon them, the lower story, with its double row of heavy logs from three and a half to four feet in thickness, is regarded of sufficient strength to resist the field artillery usually taken with cavalry on their raids.

And that was the goal – a “keep” for an outpost garrison that would afford protection against the raider’s weapons.

Now some will point out that Forrest and Wheeler captured their fair share of blockhouses while out raiding.  I would offer that in most cases those captures involved a pause of action under a flag of truce.  Words like “… to prevent further effusion of blood” were mentioned.  So we might contend the blockhouse did indeed serve the “keep” function even if the garrison were captured.

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 65.)

Fortification Friday: Mahan’s American Blockhouse

One of the themes I’ve worked in this series of posts is a comparison between pre-war and post-war manuals. In small ways, the comparison points to the influence of wartime experience in the practice of military science… at least in the American army.  Another such example is the use of blockhouses.  Pre-war, Mahan discussed blockhouses in relation to field fortifications as a safety redoubt or keep.  That was not, of course, to say such was the exclusive use of blockhouses.  But Mahan simply offered less thought about the use of stand-alone blockhouses. Rather the emphasis was upon structures which would be used in conjunction with standard, conventional field fortifications to meet the traditional needs of an army on campaign. Again, this is not to say Mahan didn’t agree with stand alone blockhouses, but rather that in instructions to his students he put all the emphasis on blockhouses used as keeps within field fortifications.  So this is a “how they were taught” consideration instead of a “this is the only way they would use it” declaration. Keep that fine point in mind.

On the other hand, the American military experience, drawn out even more so by the Civil War, included the need for fortifications guarding rear areas.  In particular, protection of railroad lines, bridges, and other such infrastructure was rather important. And, as we know from ample examples from official records, photographs, and other sources, the blockhouse became the preferred fortification for that need.  And Mahan identified that in his post-war writings, declaring an American-ism in such employment:

American Block-House. In the more recent block-houses erected in our service for the protection of bridges, railroad stations, etc, the sides and roof … are constructed with a double thickness of logs eighteen inches in diameter, hewn to a face of eight inches where they are in contact.  The inner logs are placed upright, the outward horizontal. A space is left in the outward casing sufficient for the fire from the loop-holes made through the inner. The horizontal logs above the loop-holes are held up by short uprights, mortised into them and into those just below. The ceiling is covered with earth, as shown in the section, three feet thick at the ridge and sloping towards the eaves to about six or nine inches, where it is confined by a pole plate. The earth is protected from the weather by a board roofing.  Tin or sheet iron ventilators are made through the roofing and ceiling, and a brick flue to receive the pipe of the stove used in cold weather.

The loop-holes are nearly of the same form and dimensions of those already given.

Mahan gave us two sets of illustrations supporting this passage.  The first demonstrated the blockhouse particulars mentioned above:

MahanFig48_49_50

We see a double layer of timbers – the inner most placed vertical and the outer laid horizontally. In both cases, the logs are flattened on the sides in contact to close up any gaps.

Also notice the provisions for loopholes.  The dimensions remained the same, generally. But instead of simply carving out loopholes, Mahan suggested a more elaborate arrangement.  I’d describe this as a set of small columns between a gap in the outer, horizontal timbers.  Traditional loophole cuts are made in the vertical timbers of the interior. So there is still the double row of protection and the minimum opening required for the musket barrel.

I say a picture is worth a thousand words:

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This is a blockhouse protecting the railroad between Nashville and Chattanooga in Tennessee.

Continuing, Mahan offered more elaborate plans for these blockhouses:

Some of these structures are built in the form of a cross, consisting of a square central chamber, twenty-four feet on a side, and of four wings of the same form and dimensions when the block-house is for cannon. An embrasure is pierced in each of the three sides of each wing to serve a single gun.  The cheeks of the embrasures are faced with logs, and the mouth is secured by a musket-proof shutter with a loop-hole in it.  The embrasures are below the level of the loop-hole, allowing these to be used whenever necessary.

Though not exclusively for the employment of artillery, the implication is that guns required more space within the fort and thus more elaborate arrangements were needed.  The addition of shutters is noteworthy and speaks to the need for crew protection in the era of muzzle-loading artillery.

But where you have artillery, you must also have a magazine:

Arrangements for magazines and store-rooms are made under the floor of the block-house in the most secure parts.

Ah… OK… answers that question.

And what about the entrance:

The entrance to the block-house may be either through a postern, the bottom of which is on the level of that of the ditch, a ramp leading from this level outwards, a door properly secured, and steps, forming the inner communications; or it may be arranged as shown in Fig. 51, 52, with a plank thrown across the ditch on the same level as the natural ground, the entrance to the door being masked by a double stoccade, leaving the same passage-way as that of the doorway.  Loop-holes in the door and sides of the building sweep this passage.

And here’s the referenced figures:

MahanFig51_52

Also note in this set of figures, and that above, the berm built up against the blockhouse.  As discussed earlier, this improved the defensive quality of the work, particularly against artillery.  Furthermore it prevented the enemy from hiding under the loopholes.  We don’t see that in the wartime photo above.  I’m of a mind we are seeing a blockhouse in the final stages of construction, rather than some flaw in the engineer’s design.  Other wartime photos show earth banked up against the blockhouse:

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This is another Tennessee blockhouse.  Wonderful details to consider – the loopholes and the entrance stand out nicely.  And consider the figure offered by Mahan (post-war) to illustrate the two story blockhouse:

MahanPage65Fig1_2

Can you find a better match? That’s great stuff!

Another illustration Mahan provided in the post-war manual detailed arrangements for the loopholes.  So while he “said” such was “already given,” teaching after the war he saw need to elaborate.  We’ll look at that next.

(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 62-3.)

Fortification Friday: Mahan’s suggestions to improve blockhouses

When it came to fortifications, Professor Mahan held to the “you can never do enough” principle. As we’ve discussed in preceding posts, in a field fortification the blockhouse was the keep, or last point of defense for the garrison.  It was to be outfitted in such a way that the garrison could cover the parapet with fires.  And interior arrangements would grant ample space for handling weapons.  But more could be done to improve the blockhouse.  First and foremost, one should surround the structure with a ditch:

The block-house is surrounded by a ditch, similar to one used for a defensive stoccade. A strong door is made in one of the re-entering angles, and a slight bridge leads from it across the ditch.

We saw this arrangement along a profile line in the illustrations:

PlateVIFig43

Somewhat analogous to the ditch-parapet in profile, with the blockhouse structure itself being the parapet.  Notice the glacis in front of the ditch.  And also the palisade in the ditch to the left. And we have those heavy doorways, conforming to the structure prescribed for outlets.  It’s all coming together here for the keep.

But we were still not finished making the blockhouse unassailable. There was more that could be done.  Perhaps a structure on top from which the garrison could fire down onto the attackers?

It has been proposed to place a slight parapet of earth on top of the block-house.  It is thought that this accumulation of earth would be too heavy for the timbers, independently of leaving but little space for the defense.  Perhaps a better arrangement might be made on top, similar to a defensive stoccade, the uprights being secured at bottom, between two pieces resting on the top pieces, and held firmly by an arrangement of riband pieces and braces.

Mahan’s method would provide a lightweight structure, sufficient to stop musketry.  Artillery, though, might turn that blockhouse parapet to splinters. So, any suggestions to counter that?

It has also been proposed to place the the interior and exterior rows of uprights three feet apart, and fill in between them with closely packed earth, for a defense against artillery.  This method has been tried, and was found to be less solid than the one here laid down, independently of being more difficult to construct.

Recall, the guidance stated earlier was to use two thicknesses of twelve inch timbers for defense against field artillery. This was seen as more sustainable, with less physical footprint, as what would amount to packed earth at steeper than a natural slope.

Other techniques to improve the blockhouse included methods to make the enemy’s closure even more difficult.  “The top pieces should in no case project more than twelve inches beyond the sides, to admit of logs, &c., being rolled over on the enemy.”

And if that was not enough, one could stack up a second floor (somewhat as we saw from the colonial era Fort King George):

The block-house is sometimes arranged with two stories, the corners or the sides of the upper story projecting over the sides of the lower. Either of these methods is sufficient for the defense of the lower story; but the first is the best to procure a fire in the direction of the angles.  It can only be used, however, as a defense against infantry.

When artillery cannot be brought to bear against the top of the block-house, it may be constructed like an ordinary floor, and be covered with nine or twelve inches of earth to guard against fire.

Of course, where artillery might be brought to bear against the blockhouse… well let us just say a lower profile was preferred.

But, Mahan was tapping on this point about “places where artillery will not be” for a purpose.  He was looking toward the “west” of that era:

The application of wood to the purposes of defense is one of paramount importance in our country.  A block-house, surrounded by a defensive stoccade, is impregnable to the attack of infantry if properly defended, and is therefore peculiarly suitable to either wooded or mountainous positions, where a train of artillery cannot be taken without great labor, owing to the impediments that may be thrown in its way, by rendering the roads impassible from obstructions easily obtained.

Of course, just couple of decades after Mahan’s writing armies backed with steam power (rail and river) were able to overcome many of the natural impediments of the Western Theater.  Still, the suggestion held some merit further to the west, where most potential adversary possessed only light artillery. Oh, and the mountains were much higher.

And Mahan also saw an application for the blockhouse in the east, where pre-war thinking was focused on attackers that arrived by ship:

In positions covered by extensive earthen works, such as those that would be required for the defence of the towns on our sea-board, and which would be occupied during a war, a defensive arrangement of the barracks for the troops, so that they might serve, in case of the main works being force, as rallying points, under cover of which the main body of troops may retreat with safety, is a subject that commends itself to serious attention of the engineer.  From the details already entered into, an efficient combination for this purpose will suggest itself to the reader, without entering farther into particulars.

Such arrangements might have been of use for Confederates defending the Carolinas, Georgia, Florida, and the Gulf Coast.  We might contend the inner defensive lines at Charleston served the purposes of a keep in this regard.  But here again technology had rendered the blockhouse less useful.  By 1863, Federals could bring to bear rifled artillery of the largest caliber wherever they might encounter a blockhouse.

Yet, while we can say rifled artillery and means to transport such weapons rendered the blockhouse less desirable, we still can point to widespread use of such structures throughout the Civil War.  In particular along lines of communication.  In those rear areas, raiders were not likely to bring more than a handful of artillery pieces.  And that experience lead to some shifts in the instruction about blockhouses as field fortifications… which we shall discuss next.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 64-6.)

 

Fortification Friday: Loopholes and Vents for the blockhouse

Last week, we discussed the layout and arrangements for the blockhouse, when used as part of a keep in the interior of a fortification.  An important requirement, if the keep was to function as intended, was the ability of the defenders to fire out of the blockhouse.  Just as with building the banquette, embrasures, and other arrangements on the parapet, such arrangements within the blockhouse necessitated attention to details. And those details come in the form of loopholes and vents, as Mahan would write:

The loop-holes are three feet apart; their interior dimensions are twelve inches in height; and eight inches in width for sides twelve inches thick; and twelve inches square for sides two feet thick. The width on the exterior, for the same thicknesses, will be two-and-a-half and four inches.  The height of the loop-hole on the exterior will depend on the points being defended; it should admit of the musket being fired under an elevation and depression. The height of the loop-hole above the exterior ground is six feet.

The visual you should have in mind is that of an aperture which is small on the exterior but larger for the interior.  This would allow the defender to train the musket across a wide arc, as well as providing for elevation and declination.  I don’t like mixing field fortifications with permanent fortifications, but in this case the application is along the same lines.  So consider the loop-holes here at Fort Pulaski, to the right of an embrasure:

Ft Pulaski 3 Aug 11 1346

In this case, there was need for the muskets to cover two zones.  So we see two loopholes incorporated as a pair.  Note the placement of stone slab above and below to strengthen the loophole structurally. Something not needed within the wooden blockhouse – simple cut outs within the timbers usually sufficed.

But the major difference between the blockhouse and brick fortification’s loopholes is the height.  Mahan specified only twelve inches for the blockhouse in a field fortification.  Those at Fort Pulaski are two feet or so.

Another aspect to keep in mind is the depth of the wall.  As the wall became thicker, the loophole’s lateral dimensions, particularly interior, increased.  Geometry at play here, as the musket would need more clearance on the interior as depth increased.

Mahan did not directly discuss interior arrangements for the artillery’s embrasures.  Partly, I think, as such an allocation would have pulled valuable cannons off the fort’s primary defensive line to that of the secondary or even tertiary defenses. But, we can deduce such arrangements would match those described for embrasures through the parapet.  In short, a larger loophole… which is what we see to the left of the photo above.

All this is good thinking.  But we also have to keep in mind the by-product of firing any weapon.  In order to push the projectile out of the barrel, firing of the powder creates gasses. That foul air is not an issue out in the open or on the parapet.  But in the enclosed space of the blockhouse, there is need to expel the gasses:

Vents for the escape of the smoke are made over each loop-hole, between the cap-sill and the top pieces.

Moving to another location in Fort Pulaski, we see a vent above one of the other embrasures:

Ft Pulaski 3 Aug 11 1388

See the weathering on the paint?

Mahan does not provide much information on constructing vents for the blockhouse. These could be vents between the ceiling and wall. Or vents incorporated in the wall itself.  To maintain integrity of the structure, in terms of defense, those vents were best created using an interior angle.  That would allow gasses to vent.  But water… or things the enemy might want to push inside… would be restricted.

From there, Mahan gave brief descriptions of the camp bed (which we noted served as the banquette inside the blockhouse), racks, and other storage arrangements.  But with that he left the interior arrangements.  Instead he turned to an external details.  We’ll look at those next week.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 64.)

Fortification Friday: Square, rectangle, or even a cross – blockhouse forms

Last week, we introduced the blockhouse as an interior structure, perhaps better classified as a facility, within a field fortification.  Allow me to stress again, the context of Mahan’s writings in “Field Fortifications” about blockhouses was scoped to discuss the use of those sort of structures in conjunction with larger works.  It was not to say blockhouses would always be used as such, nor to dismiss other sorts of employment of that fortification type. This particular Mahan lesson (of which there were many, across several manuals, as we must recall) was focused on building a “keep” so the defenders might “keep” something valuable… their lives in the event all was lost.

Having discussed the concept and general layout of the blockhouse, Mahan turned to particulars:

With regard to the details of the construction, the timber for the sides should be twelve inches thick, to resist an attack of musketry, and to resist field-pieces, two feet, in which case the sides are formed of two thicknesses of twelve-inch timber. If the timber is placed upright, each piece should be let into a mortise in the cap-sill; and every fourth piece of the top, at least, should be notched on the cap-sill, to prevent the sides from spreading out.

This would form, in essence, the walls of the blockhouse.  Notice the prescribed thickness, in regard to the expected threat – be that musketry or artillery.  I would add that with the introduction of rifled artillery, the two foot thickness was insufficient.  But there begins a point of diminishing return. How much more timber should one add to the blockhouse, thus subtracting usable interior space, in order to defend against an Ordnance or Parrott rifle?  Ah… a question best addressed when we consider the post-war manuals!  So let’s hold that thought.

I do wish Mahan had included a good illustration of the proposed arrangement of timbers. And I’ve not located any other contemporary illustration to serve.  But the general idea is apparent… perhaps for generations who suffered the splinters from Lincoln Logs, if not so much for those of more recent times and their Lego bricks.  We will revisit the arrangement of timbers in the walls for the post-war manuals.

Moving forward, we need to consider the layout of those walls and how best to arrange the blockhouse in order to meet requirements:

The plan of the block-house must conform to its object generally; it may be square or rectangular.  If flank defenses are required, its play may be that of a cross. The interior height should not be less than nine feet, to allow ample room for loading the musket; this height will require that the timber of the sides shall be twelve feet long, in order to firmly set in the earth.  Sometimes a ground sill is placed under the uprights, but this is seldom necessary.  The width may be only twelve feet in some cases, but it is better to allow twenty feet; this will admit of a camp bed of boards on each side, six-and-a-half feet wide, and free space of seven feet….

So the layout, as seen from above, could be the square form familiar to us from the playsets of yore.  Or could be extended or expanded to use other layouts as tactical needs demanded.  The layout tended to employ right angles, however.  We look back at Figure 44, which is somewhat a cross, in plan:

PlateVIFig44

Notice how the dimensions are governed somewhat by the need to provide space for handling muskets.  Form will follow function.  The most important quality of the blockhouse, as a keep, is to allow the garrison to create a pause in the action, should the parapet be lost.

But “camp bed”?  Yes, that implies a place to sleep. But it was also a defensive arrangement.  “The camp bed serves also as a banquette; it is placed four feet three inches below the loop-hole, and has a slight slope of about eight inches inwards.”  Notice how the interior arrangement is to provide, in terms of wall to wall floor space, for a 6 ½ foot wide camp bed on each side with open space for seven feet between.

Now everything thus far has implied the garrison would only have muskets in the blockhouse.  Let us make arrangements, then, for artillery:

If cannon is to be used for the defense, the width must be at least twenty-four feet; this will allow eighteen feet for the service of the gun, which is generally ample, and six feet for a defense of musketry on the opposite side.  A greater width than twenty-four feet cannot well be allowed, because the bearing would be too great between the sides for twelve inch timber; and even for a width of sixteen feet it would be well to support the top pieces, by placing a girder under them on the shores.

Basically, bigger guns require more space.  So we adjust the arrangements.  But there is a physical limit as to how much more space is allocated.  If a really large blockhouse were built, it would require substantial structural reinforcement.  Better to stick with a single cannon per side, if used at all.

Since these arrangements place emphasis on affording space to handle weapons, be that musket or cannon, we need to discuss the loopholes in detail.  We’ll turn to that in the next installment.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 63-4.)

Fortification Friday: Blockhouses as Safety Redoubts in the Fort

When I say “blockhouse” many of you might be thinking about favorite childhood playsets:

OLYMPUS DIGITAL CAMERA

Certainly suitable for the defense of the bedroom in the face of the elite Confederate Plastic Brigade, or perhaps the indigenous Plastikawi tribe.  But… something that could not hold against the Green Army Men armed with bazookas and flamethrowers.

Kidding aside, the playset fort is pattered after real structures from American history.  The blockhouse was not unique to America, as it was a form brought over by Europeans.  However, the blockhouse became the preferred fortification on the North American continent from colonial times right up to the 20th century.  Blockhouses work well in situations where the enemy is unlikely to possess anything larger than light artillery.  The interior of the blockhouse was easily adapted into living quarters.  Conversely, living quarters (houses) might be easily adapted into a blockhouse.  Those, and other qualities, made that form of fortification popular on the frontier.

The popular image of a blockhouse is something made of wood.  But stone, or even adobe, might be used instead.  Since wood was in abundance on the early American frontier, we tend to see a lot of structures like this one:

Ft King George 3 Aug 11 1273

This is recreation of Fort King George, Darien, Georgia (a place with many, many layers of history).  In this particular case, the blockhouse served several roles – a high observation platform over the marsh, a platform for covering fire to protect approaches to the fort, and, in the event the works were overwhelmed, a final defense for the fort’s garrison.

It is that last function that Mahan had in mind when considering interior arrangements for field fortifications.  Blockhouses were a structure that could be used for what he called “safety redoubts”:

Safety Redoubt.  In enclosed works a place of retreat, into which the troops may retire in safety after a vigorous defense of the main work, will remove the fears of the garrison for the consequences of a successful attack of the enemy, and will inspire them with confidence to hold out to the last moment.

This interior work, which may be very properly be termed the keep, can only be applied to works of large interior capacity.  It may be formed of earth, or consist simply of a space enclosed by a defensive stoccade, or palisading.  In either case it should be about four feet higher than the main work, to prevent the enemy from obtaining a plunging fire in it from the parapet of the main work.

Let us pause here before going to Mahan’s formal introduction of the blockhouse.  This “hold out to the last” is a notion steeped in 19th century presumptions about how a siege would play out.  A garrison “holding out” would force the enemy to make a direct attack on the parapet… in other words, to get up close, personal, and… well… very violent with the defender.  And in that violent melee, the defender was not exactly in a position to call a “time out”.

The safety redoubt, or keep, was a place to retreat and, more importantly, force a pause in the action.  And from the keep, within that pause, the defender might negotiate a cessation of the fight, with honor.  Thus we see how that might allay fears of “consequences” for the garrison.

That in mind, Mahan offered his preference for the keep:

The best arrangement for the keep is the construction termed the block-house. This work is made of heavy timber, either squared on two sides or four; the pieces which form the sides of the block-house are either laid horizontally, and halved together at the ends, like an ordinary log-house, or else they are placed vertically, side by side, and connected at the top by a cap-sill. The sides are arranged with loop-hole defenses; and the top is formed by laying heavy logs, side by side, of the same thickness as those used for the sides, and covering them with earth to the depth of three feet.

Mahan offered this figure as an example of a blockhouse:

PlateVIFig43

This perspective is looking at the blockhouse along with a cross section of adjacent works and structures.  Rather busy.  This section is along the line a-b from Figure 44:

PlateVIFig44

The combined caption reads:

Figs. 43,44. Shows the plan and section of a block-house of upright timber.  The plan is made to exhibit a portion of the top complete; the timber covering the top; the arrangement of the cap pieces; a plan of the loop-holes; and a plan of the camp-bed. Fig. 43 exhibits, in a like manner, a cross section of the block-house and ditch; with interior and exterior elevation.

We will go into the particulars for construction in later posts.  What is important to identify here is the functional nature of this blockhouse.  Just as with the colonial-era Fort King George, we see a blockhouse adjacent to a ditch and other defensive structures.  One might say the blockhouse filled up the fort’s interior.

For an attacker, this presents a serious tactical problem.  One might defeat the defender on the parapet.  But the parapet would be a dangerous place to make a living with the blockhouse overlooking all. So you see where a “pause” would be in order.

Keep in mind, within this discussion of keeps, Mahan was not stating or suggesting that blockhouses only be constructed within and in conjunction with elaborate field works.  Rather that he offered that a blockhouse was a structure that served well as a keep inside a larger set of works.  We see that usage applied by his students during the Civil War.  Looking back again to Fortress Rosecrans:

FortressRosecrans

We see Redoubts Schofield, Brannan, T.J. Wood, and Johnson within the interior.  One wartime report described the arrangement as, “… strong against attack, being defended by large keeps, which deliver their fire upon every part of the interior.”  I would further add that most of the lunettes on the perimeter of this vast fortress included blockhouses.  So there were multiple “keeps” within a depth of the defense.  Keep in mind the scale of this fortress.  The safety redoubts, named above, were armed with 30-pdr Parrotts and 8-inch siege howitzers.  The Confederates would need to bring a large amount of iron in order to suppress the fort’s garrison.

But the size of this work was perhaps its weak point.  After the Army of the Cumberland moved further south, through the summer of 1863, there the need to keep this fortification in order was taxing, in terms of manpower. An 1865 report suggested all be reduced to simple blockhouses covering the bridge and depots.

That circles back to the point about blockhouse usage.  As said before, Mahan was not suggesting the only place to use a blockhouse was as a fort’s keep. But as his text was focused on field fortifications, the focus was on that function.  We will see blockhouses enter the conversation in regard to detached defenses in particular.  Furthermore, the post-war instructions would place more emphasis on the detached, singular blockhouse.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 62-3; OR, Series I, Volume XLIX, Part 2, Serial 104, page 502.)