The Torre del Oro (English: "Tower of Gold") is a dodecagonal military watchtower in Seville, southern Spain. It was erected by the Almohad Caliphate in order to control access to Seville via the Guadalquivir river. Constructed in the first third of the 13th century, the tower served as a prison during the Middle Ages. Its name comes from the golden shine it projected on the river, due to its building materials (a mixture of mortar, lime and pressed hay). The tower is divided into three levels, with the third and uppermost being circular in shape and added in 1769. The Torre de la Plata, an octagonal tower, is located nearby, and is believed to have been constructed during the same era.