A Cidá do Castro also known as Castro San Millao, is located in Cualedro (Ourense), its history and first inhabitants go back to the megalithic age, counting on its borders with 50 archaeological sites, seven of which are forts. Outstanding example, declared a national monument, is Cidá do Castro, it preserves the remains of a fortified precinct in the Iron Age, dated between II and IV centuries A.D.
It consists of a moat eight metres high and between 6 and 10 metres wide. The most outstanding elements are the impressive cyclopean defensive walls, reinforced with a plan area made of large prism-shaped vertical stones driven into the ground which height reaches seventy centimetres. Its size and defensive function, typical of the forts in the southern part of Galicia, became it in an impregnable fortress for its time. The fort with north-south orientation, seems to have an egg-shaped floor and presents different precincts at different levels, all independent of each other.
López Cuevillas and Taboada Chivite start the first studies on the settlement in 1953, being the beginning of the first excavation in 1954. Rodríguez González and Fariña Busto take the studies up again in 1982 and 1983 with cleaning and excavation works. During these investigations were found remains of metallic utensils, circular mills and decorated ceramics.