The Roman Turris Sillae, built on the hill of Siellas, was the bulwark of the defensive line of the Duero during the Reconquest. In 1262 it received its charter from Alfonso X the Wise. The town began to be favored by the royal family and nobility, above all after Alfonso XI built a palace (1325). In the 15th century the town hosted several meetings of the Cortes. During the skirmishes between Henry IV and the nobility the city supported the monarchy, and again during the clashes between the Catholic Monarchs and Joanna La Beltraneja in 1476.
The Catholic Monarchs signed the Treaty of Tordesillas with the Portuguese crown in 1494, which established the line dividing the globe between Spain and Portugal for colonization purposes. This especially affected the Portuguese and Spanish colonization of the Americas, and placed the name Tordesillas in history worldwide ever since.