This stronghold was built by Alfonso VI on a hill high above the city on the site of the ruins of a Muslim fort. It is rectangular in structure and flanked by four square towers. The facade presents rows of balconies and windows and a walkway with banisters. Alsonso de Covarrubias was responsible for the picturesque gateway. Nowadays, it is not only the home of the Museo del Ejército (Army Museum), but also houses the Regional Library which holds important collections and archives. The library has all the latest technology and provides computer terminals for Internet access for library users.