It is today the site of the city's Army Museum. Its construction dates from Roman times. During the reigns of Alfonso VI and Alfonso X 'the Wise' it was rebuilt and became the first example of a square fortress with towers on the corners. It was once again rebuilt under Emperor Charles V, on this occasion by the architect Alonso de Covarrubias. The façades are Renaissance in style, and it has towers and crenellated defences according to a preliminary design by Alonso de Covarrubias, subsequently completed by Juan de Herrera. After the last reconstruction it became the site of the Army offices and museum.