Well it doesn't have to be a gangbanger fortress. You could be in charge of a mafia and have a regular mansion or you could customize it to make it your own. Just a huge base of operations for your group, be it a gang, mafia, or some other organization. If some sort of gang customization comes into play, the fortress could be its main hub where you change the gang's options and settings, e.i. their weapons, colors, cars, ect.