Strongholds are an older part of the mod that was removed in the first 1.7.10 update, and have since returned in v1.1. Strongholds are fairly different from their old style, now focusing more on vannilla mobs from vannilla spawners rather than Cq mobs and Cq spawners (although there's still plenty of that.) They are made up of a bunch of randomly generated rooms, with a much more open feel than earlier versions, due to less constrained rooms that connect more openly. They are the only dungeon that use the loading-bar type generation. There is also a desert varient that is populated by mummies.