From A Wiki of Ice and Fire
The Five Forts are massive fortresses along the northeastern boundaries of the Golden Empire of Yi Ti, situated between the Bleeding Sea and the Mountains of the Morn. Northeast of the Five Forts are the Land of the Shrykes, K'Dath, Bonetown, the Grey Waste, and the Cannibal Sands.
The forts' walls are slabs of fused black stone almost a thousand feet high. Some think the walls are Valyrian in origin, although there is no record of Valyrians having visited the area and they even predate the rise of the Freehold. Each fort can house ten thousand men.
The Five Forts predate the Golden Empire of Yi Ti. Some suggest they were built by the Pearl Emperor at the time of the Great Empire of the Dawn to defend against the demons of the Lion of Night. They currently guard the Golden Empire from raiders out of the Grey Waste.