Located next to the Church of Santa María, it is one of the few castles preserved in Cantabria. Together with the walls, of which few remains remain, it was a key point for the city's defense.
It was probably built as a defensive element in the late 12th and early 13th centuries. It is small, measuring 25 by 15 meters on each side, with a trapezoidal floor plan, cylindrical corner towers, and a triangular projection also topped with a cylinder. Inside, there is a barrel-vaulted room, which also houses a reproduction of the Roman milestone of Nero Claudius. Various events and exhibitions are currently held here.
In 1853, the lighthouse was built in the southeast tower, with a circular layout and made of ashlar, making it one of the oldest in the region.
Along with the entire Puebla Vieja of Castro-Urdiales, it was declared a Historic-Artistic Site in 1978.
