The castle is surrounded by a still complete wall ring, which is mostly visible to the south and hits the rock of the mountain. The castle complex is angled and spans the entire mountain bridge in an east-west direction over a width of about 100 meters. In a north-south direction it is only about 25 meters. The interior buildings with different levels and floor heights, of which seven separate houses are preserved today, are connected by a small road. The western side faces the sea. Access is from east to north via a steep ramp leading to the concierge in the north center, whose entrance could be flanked and controlled by buildings from three sides in a small courtyard. To the north of the castle, which also points northwest and with a small round tower and protected platform, the remains of a kind of external bailey (or kennel) have been preserved. A rainwater cistern with a small covered well in the eastern castle area indicates the difficulties of water supply.
The place and perhaps a first fortification were founded in 1102, the castle as such only in the thirteenth century (probably around 1270) by the noble Genoese Doria family as “Castel Genovese” or “Castrum Januae” with great strategic importance. Until the fifteenth century, the city and the castle were the seat of the Doria and served to dominate the possessions in northern Sardinia. About Castelsardo there was an influx of Ligurian and Corsican population groups.
The Doria built the place and the castle as a city republic, according to the Ligurian model with its own code of law, the Statutes of Galeotto Doria and the motto “Pax et Bonum Rempublicam conservatore”. The Dorias later joined with the Giudicato of Arborea, the last most powerful of the Sardinian judges by marriage of Brancaleone Doria with the Sardinian heroine Eleonora d’Arborea, probably not least to insist on the intrigue between pope and emperor, after the pope in 1295 the Crown of Aragon under James II of Aragon.
Conquered in 1448 by the Spaniards of the Crown of Aragon as the last Sardinian territory, the castle was then called “Castel Aragonese”, the place became a royal city (“Royal city of Sardinia”) until 1479 (later Oristano). Castelsardo (castle and place were not distinguished) remained until 1586, until the construction of the Cathedral of Sant’Antonio Abate, episcopal seat.
From 1767 and with the dominion of the House of Savoy, the castle and the city received the current name Castelsardo. For centuries the castle was occupied by the military; until a few decades ago it still served as the Carabinieri barracks.