The real origin of Tordesillas is not known. Townships from the second age of the iron have been found. Probably, Iberian and celtic tribes were living in caves from the VI century to the III B.C.
They were placed on Vacceo lands. Some authors believe it was Sarabis’ town, some others that it was Acontia’s.
Three origins have been given according to the etymology of "Tordesillas". Some authors talk about a roman consul called Quinto Cecilio Metelo as the town’s founder in the 83rd B.C. The consul built a tower in the roman dictator Lucio Cornelio Sila’s honour. It could be the one which is called "Torreón de las Acercas" or "Torre de Sila", the ancient tower in Tordesillas (63rd B.C.). The name of Tordesillas is probably to derive from "Turris-Syllae" which means Sila's fortress, to Turris-Sylana. On the other hand, it is believed that the tower is only a group of stones that belonged to the ancient wall.
Another version says that the name has its origins in the age of the Arabian authoriry, and comes from the word "Thor Shilah", that means fortress of the shilanes; Arabic tribes settled themselves in the region in the 8th century.
"Oter de Sillas" or "Oter de Cillas" is considered to be the origin of Tordesillas by other authors, the word "oter" means Otero (in spanish, lonely hill over a plain) which probably refers to a high place, and Tordesillas is placed on a high land. The term "Sillas" would come from the scenic overlook with stones in the form of ashlar blocks* located around the river. According to other versions the word "cilla" would mean cellar, because of the natural cellars founded near the river.
*ashlar block: Large rectangular dressed stone sculpted to have square edges and even faces, used in the construction of many old buildings.