When the Great War came in 2077, it was one of the highest-profile targets in the area, suffering a number of direct nuclear hits that obliterated much of its surface facilities, leaving a gaping crater and concrete ruins behind. The nuclear detonation did not destroy the bunker's underground installations, however, which was likely the main reason it was secured by Talon Company mercenaries and used as their primary base.
Its missile silo doors reside in a ruined building. As the building has a roughly circular pattern of destruction, which centers on the silo doors, one can gather that the building itself collapsed when a missile ploughed through it, most likely at the start of the Great War.
The bunker itself is extensive and is accessible from the ammunition storage, a manhole inside a commander's tent, a bunker entrance just above the commander's tent, and a metal door down some steps just to the right when entering Fort Bannister through the south gates.
The underground levels of the fort are crawling with hostile Talon Company mercs who are all very heavily armed. If the Lone Wanderer's combat skills are high enough to clear the place out, the area contains a staggering amount of valuable hardware to scavenge. The mercenaries here have a wide range of armaments, including assault rifles, Chinese assault rifles, combat shotguns, laser rifles, flamers, missile launchers and 10mm SMGs.
In the commanding officer's quarters are Commander Jabsco, the leader of the company, a Mister Gutsy (though the Mister Gutsy model may be found in a dormant state, and is likely activated once the player character attacks Jabsco) and two automated turrets. However, the first of these turrets (the one upstairs) can be either deactivated or frenzied via a locked terminal just outside the commander's personal quarters. Although the frenzied turret will track and attack Commander Jabsco, it rarely fires at him when he has a melee weapon armed.
The commander himself starts off with a missile launcher, but once he runs out of ammunition for that, he will either attack with his Occam's Razor, pull out his Chinese pistol or rush downstairs and grab any weapon in the nearby safe. There is also a chance that once the player character attacks the commander, more Talon Company mercs will spawn behind them. He will actually call out for assistance when the conditions for reinforcement have been met.
In Commander Jabsco's personal quarters, there are four generators which will explode when shot or sufficiently damaged by explosives.
With the Broken Steel add-on installed, the player character should find several Aqua Pura pack brahmin near the tents. When searching the tents, they should also find Brotherhood Paladin Jensen tied up. He tells the player character that the Brotherhood is tired of having their Aqua Pura stolen by Talon Company, and intends to get some payback - Jensen is the scout for the operation.
If the player character frees him, he'll charge down into the fort, quickly getting himself killed by the awaiting turret and the missile-launching Talon Company merc. Soon thereafter, two to four more Brotherhood of Steel knights and paladins will run in after him. The Talon Company defenses will shortly prove too much for the Brotherhood, and most of them will likely die. Alternatively, it is also possible that the knights will defeat the mercs and occupy Fort Bannister if the player character decides to assist them. It will become another BoS outpost soon after.
It is also possible to run into the Brotherhood's Alpha Squad (just south of the fort), which will soon assault the fort, before freeing Jensen or even approaching the fort. They can be convinced to assault the fort before freeing Jensen, who will be freed at some point during the assault.
After The Waters of Life quest, an Enclave research outpost can be found near the large crater, with a field research terminal containing their objective in the area (gathering samples). The camp is occupied by three Enclave soldiers, one Enclave officer and one Enclave scientist. There is also an Enclave turret. There are three Enclave crates and in one of which one can store gear (the crate at eye level, behind the computer terminal). In the crater there are eight alien power cell cartridges underneath the overturned truck. There is a constant low level of radiation (+1/2).
Fort Bannister main
- In an Average locked room at the bottom of the concrete stairs, there is a copy of Big Book of Science, a Fat Man and one mini nuke. From the southernmost (Very Easy locked) entrance to Fort Bannister Main, just follow the metal steps up to the top, continue through the doorway, to the left, down the concrete stairs and behind a locked (average) door.
Fort Bannister bunker
- A copy of Guns and Bullets is found on top of a gun cabinet inside the easternmost room of the bunker area of Fort Bannister. The bunker has a very hard lock or can be entered via the commanding officer's quarters.
Commanding officer's quarters
- A copy of Nikola Tesla and You can be found atop the lowermost covered walkway in the missile launch room, next to a Ham radio and an ammunition box. The missile launch room is the first you enter after the sewer entrance from the outside.
- The unique combat knife called Occam's Razor and unique combat helmet called the Talon combat helmet are carried by the Talon Company leader, Commander Jabsco, who is found in the southernmost room.
- A Nuka-Cola Quantum behind a footlocker under the stairs in the southernmost room.
The frag mines surrounding the downed section of the chain-link gate on the eastern entrance are not armed.
Fort Bannister appears only in Fallout 3.
If the player kills everyone inside the fort while the Brotherhood are still hiding, there is a chance the Brotherhood will instead make the tents a base, rather than the fort. Even if you don't kill anyone at all inside the fort, they will still occupy the tents. Manage to keep everyone alive on the Brotherhood team, and they still leave the fort after the assault. [verified]
- It's also possible that if the player kills everyone inside the fort while the Brotherhood are still hiding, the Aqua Pura pack brahmin and Paladin Jensen will not spawn, nor will the rest of Alpha Squad, even after completing all Broken Steel main quests. [verified]