We’ve heard Dennis Hart Mahan’s lessons on simple “intrenchments” and thus far covered open works and closed works. Of the latter, the subset included redoubts, star forts, and bastion forts. The last of those three is, considering the diagram offered to illustrate, somewhat where we started:
I’ve traced the bastion fort’s plan in blue for clarity. Mahan’s textbook diagram left out the third bastion of this plan, so I’ve added dashed lines where the walls would extend further. From there you’ll have to use your imagination. Shouldn’t bee too hard, as this would be a common plan most have seen applied to forts of the era.
Mahan introduced the bastion fort with a hand of preference:
The bastion fort satisfies more fully the conditions of a good defense, than any other work; but, owning to the time and labor required for its construction, it should be applied only to sites of great importance, which demand the presence of troops during the operations of a campaign.
Before we go too much further, let’s go back and note what a bastion was, formally speaking. We saw the term used as an alternate label for the lunette. A bastion, be it the detached variety (which I’ll call a lunette for better distinction) or an “attached” specimen as part of a bastion fort, includes two faces and two flanks, thus including a salient angle.
And, as with all these fixtures in the fortifications, there were variations of bastions to describe… and denominate. We’ve mentioned lunettes as detached bastions. Beyond that, there is reference to empty bastions, full bastions, flat bastions, demi-bastions, and tower bastions.
Empty bastions were constructed so that the interior of the bastion was at the same level of the interior of the larger fortification. Full bastions, on the other hand, had elevated interiors. One immediate application for a full bastion was to afford artillery a clear, elevated line of fire out of the fortification.
Off hand, we identify bastions as a feature for the corners of fortifications. But, as mentioned in the discussion of redoubts, sometimes a bastion was needed along the side of a fortification. This was known as a flat bastion. I remember it as a bastion attached to the “flats” of the fortification plan.
Demi-bastions, or half-bastions, are somewhat as the name implies – a plan where one side of the bastion contains a face and flank, while the other side is just a straight line back to the base line of the fortification. Somewhat asymmetrical, yes. But more common than one might believe as half-bastions allowed engineers better adjustments to local conditions. However, I would point out that Mahan didn’t recommend these half solutions.
And tower bastions? Originally this term applied to masonry fortifications, specifically a structure built that included gun embrasures and interior galleries. But sometimes the term was applied to earthen works where the bastion had elaborate gun positions, traverses, and bombproofs added. It’s not technically “correct” but who am I to argue with a 150 year old account?
So, you see the engineer had a lot of choices in regard to bastions. And these being Mahan’s preferred type of “intrenchment,” there was a great deal of emphasis placed on learning how to plan a bastion fort. We’ll walk through that in detail in future posts. The larger point, making this a good place to pause in the discussion, is that Mahan impressed upon his students the favorable aspects of the bastion fort. He followed that up with deliberate instructions for building those bastion forts correctly. His students were not a bunch of “nobody” cadets, but rather the fellows that ended up in charge of “the show.” Little wonder we see a lot of bastions built from 1861 to 1865.
(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 14.)