On a prominent spot broken off from the San Martín moor, rise the ruins of the Astudillo castle. Although its origin could date back to the 10th century, there is no documentary evidence of its existence until the 11th century. It was possibly erected to reinforce the important fortress of Castrojeriz and would play an important role in the confrontations between Ferdinand I and Vermudo III for control of these lands near the Pisuerga River, whose course separated the kingdoms of Castile and León. It also suffered the turbulent years of the reign of María de Molina (lady of Astudillo) and Juan II (who spent long periods in this town).