Iraj Castle is a fortification built in 4th or 5th century AD in the central region of Ray, Sasanian Empire, near modern-day Asgarabad-e Abbasi, Iran. The monument is known for its peculiar design: large defensive walls with embedded rooms and arches, with towers at regular intervals, enclosing a vast empty interior of 190 hectares (470 acres).
The design is similar to several other Sasanian campaign bases. The permanent service members supposedly occupied the walls and the rooms inside it, while the empty interior would be occupied by tents or used for training or military assemblies. It probably served as a central military base for operations elsewhere, especially against the incursions of nomadic nations in the north.