The phrase "old fort" refers to a very old and ancient structure that was built to protect people from enemies or invaders. It is a fortified place where soldiers used to live and defend their community. Full definition
WHO DOES DEFENSIVE DRIVING ONLINE IN OLD FORT NORTH CAROLINA? (drivinguniversity.com)
The Indian state of Rajasthan is full of old forts and palaces, but Jodhpur's Mehrangarh Fort is truly one of the most spectacular. (aroundthisworld.com)