The Fortress is a reinforced pre-War storage facility located in the Eastern part of the Boneyard, surrounded by a moat of glowing green goo. This intimidating location is the home of the Gun Runners, and contains their manufacturing facilities, where they create high quality weapons that pack a definitive punch.
With high enough barter you can buy and sell back to the 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
This is the location of Zack, an important arms merchant in Fallout.
This is the location of Gabriel, an important figure in the Boneyard unmarked quests.