Castillo de Peñaranda de Duero is an 11th-century fortress built during the Reconquest to defend against Moorish advance, later transformed by the Count of Miranda del Castañar in the 15th century into a powerful centre of territorial control. The castle features a distinctive square-planned keep tower containing four wooden-beamed floors, surrounded by reinforced walls, semicircular towers, arrow slits, crenellations, and a moat carved from living rock. Declared a Historical-Artistic Monument in 1931, the castle's keep now houses an interpretation centre with seven museum rooms showcasing the fortress's architectural and historical significance within medieval Castilian defensive systems.