The fortress was founded by the first Marquis of Santillana, Íñigo López de Mendoza. It is built on the site of an old Romanesque-Mudejar shrine, remains of which can still be seen inside. The building has a square floor plan. It has circular towers at the corners and a polygonal keep. The castle comprises a courtyard, basement and six storeys. On the first floor there is a Gothic gallery, considered to be one of the most beautiful in existence. It is surrounded by a barbican whose arrow slits are decorated with sculpted Crosses of the Holy Sepulchre of Jerusalem. This castle belongs to the Autonomous Region of Madrid and is used for congresses and conventions.