The Fortress is a reinforced weapons manufacturing facility surrounded by a moat of toxic waste. Home to the Gun Runners, it houses much of their machinery for making high-quality weaponry as well as the weapons themselves.
With a high enough Barter skill you can buy and sell back to the Gun Runners for more than the original cost, and since the amount of money they have resets after every transaction, this can be a source of unlimited cash.
Their stock also resets after every transaction, so be careful on what you are selling or save before you buy/sell there.
It is possible to simply run through the moat in the northeastern corner, bypassing the guard at the bridge.
Surprisingly, it was never intended for the player character to talk to Gabriel, resulting in the conversations being like normal as if having a regular Intelligence above 3.