A civilization's Workers can build forts as soon as it discovers the Mathematics technology. Like most tile improvements, forts cannot be built in foreign territory. They can, however, be built in neutral territory, and of course within a civ's own territory. Forts may be built on any tile except oasis tiles. Building a fort removes any other tile improvement present except for a road or railroad. It takes Workers 10 turns to build a fort on normal speed, with the normal scaling for other speeds (15 on epic, 30 on marathon, 7 on quick).