The Cidá do Castro, also known as San Millao Fort is in Cualedro, an Orensano town whose history and early settlers date back to the megalithic age. Within its boundaries are 50 archaeological sites, of which 7 are forts. Cidá do Castro which preserves the remains of an Iron Age fort dated between 2nd and 4th century A.D., is a noteworthy example of military settlement. Furthermore, it has been declared a national monument.
It has an 8 metre high moat with a width of 6-10 metres. The noteworthy element are the impressive Cyclopean defence walls, reinforced with a flat area of large vertical stones forming a prism and pressed into the land, whose height reaches 70 centimetres. The size and defence function of this element typical of forts in southern Galicia, making it an impregnable fort for that period. The fort with North-South orientation, appears to have an ovoid layout, and has different spaces at different levels, all of which are independent.
Early studies on the settlement were begun by López Cuevillas and Taboada Chivite in 1953. The first excavation was started in 1954. Rodríguez González y Fariña Busto picked up their studies again, carrying out cleaning and excavation works in 1982 and 1983. While carrying out these investigations they located the remains of metal utensils, circular mills and decorated ceramics.