Peñaranda is one of the best conserved castles along the fortified frontier line which existed between the Christian kingdoms and the territory of Al-Ándalus during the 10th century.
Although it originally dates from the 10th century, the current building is the result of the reforms undertaken throughout the 15th century by its owners, the Counts of Miranda del Castañar. It has an elongated ground plan, with solid ashlar stone walls and a square keep rising from the centre. This structure has four storeys decorated with wooden beams and very few window openings to the exterior; on the top there is a flat terrace, battlements (the same as throughout the rest of the castle) and a gallery of machiolations. There are another three square towers at the ends, two flanking the entrance under an ogival arch. These square towers form the start of the defensive wall surrounding the town of Peñaranda de Duero, of which only the crenellated arch of Las Monjas still survives today.