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 they wield.
The player is expected to talk to the moat guard to gain access to the Fortress proper. It is possible to simply run through the moat in the northeastern corner, bypassing the guard at the bridge.
With a high enough Barter skill, items can be bought and sold back to the Gun Runners for more than the original cost. Since the amount of money they have resets after every transaction, this can be a significant source of funds. Their stock also resets after every transaction, so be careful about what items are sold or save before doing so.