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:
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.)