One of the unique features of Fort Jackson is the design of the bastions and the use of cavaliers. A cavalier is a wall that completes the polygon shape of a fort as if the bastion is not there, and overlooks the bastion. This means that if the terreplein of the bastion would fall, the attacker would be fired upon from the cavalier - an elevated position looking down on the bastion terreplein. The following sketches show this concept in plan and in section.On to the main fort, starting with the sally port. As you can see in the picture above, the sally port was flanked by two howitzer embrasures, and the picture below shows a howitzer position in the flank of the each bastion. That means four howitzers would guard the causeway and drawbridge leading to the sally port, as well as rifle and gun positions on the ramparts.
View attachment 389095
In addition to breaching the drawbridge in the "up" position, an attacker would be faced with two sets of wooden doors within the sally port. Both sets would have been three-ply doors, all three plies made of hard wood - in this case I believe them to be cypress. The outer layer would have the grain of the wood running diagonally, the middle layer on the opposite diagonal, and the inner layer the same as the outer layer. This way, someone attempting to breach the door with an axe would be able to split the outer layer with the grain, but would then be chopping cross-grain on the middle layer. In addition, every six inches in all directions would be iron studs. If the axe struck one of these studs, one of two things would happen. Best case (for the attacker) is that his back teeth would ring for hours! Worst case is that his axe head would shatter, and he'd be left with an axe handle to attempt to breach the door!
Once through the first set of doors, the attacker would find himself fired upon through loopholes in the sides of the sally port. These loopholes opened into two guard rooms, one on either side. With the brick walls and confined space, missed shots would ricochet around the area until they hit someone or their energy was expended.
View attachment 389098
Finally, when the second set of doors were breached the attackers would most likely be looking into the muzzle of a field piece standing on the parade, loaded with cannister shot.
This is the passageway that allows one soldier at a time to move from the cavalier to the bastion and back.
The mass of the bastions were casemated on the flanks and earth-filled at the salients. Each had a service magazine, and the casemated area was entered from the parade. The picture below shows the entrances to the casemates in two flanks of a bastion.
Two of the fronts of the fort face the Mississippi River, and those are the only two fronts that are casemated. They open to the parade in the rear, and with embrasures facing the river.
The large earthen mound to the left of the picture is the earthen fill around the Endicott Period battery that was poured on the parade of the fort.
In addition to the guns of the fort, there was a salient place d'armes designed into the downstream coverface that held a water battery.
In 1858, P. G. T. Beauregard upgraded the defenses of Fort Jackson. One of the principle upgrades was the construction of an external water battery upstream of the outer ditch.
This was the fort that Porter and Farragut were able to run past when they attacked New Orleans. As a result of them running past Fort Jackson, and a similar run-past at Fort Morgan on Mobile Point, the post-war Corps of Engineers decided to reevaluate how to stop warships from entering a harbor. The result was recorded in a treatise by John Gross Barnard, who stated that 15-inch and larger Rodman guns were needed to stop a steam-powered ironclad. The problem - these guns would not fit in the casemates of Fort Jackson or of many other Third System forts. With a limited budget after the war, it was decided that masonry-revetted earthworks were the best solution. The doctrine established was two gun positions, a traverse containing a ready magazine, then two more gun positions, and so on. This program, called the 1870s Modernization, was implemented at a number of Third System locations, including Fort Jackson.
The Beauregard external battery was replaced with a series of gun emplacements and traverse magazines. In addition, the two salient places d'armes were emplaced with larger guns, and the cavalier of the bastion centered on the river was used for another battery. The sketch below shows these locations.
The photo below shows the entrance to an 1870s magazine. There were two-gun emplacements on each side of the magazine. These are quite easy to see in the remains of the 1870s battery.
In the 1880s, the development of breechloading, steel artillery launched the Endicott Period of concrete fortifications. At Fort Jackson, a battery was poured on the parade of the old fort and another on the upstream salient place d'armes. The drawing below shows these modifications.