Torre del Oro
The Torre del Oro (English: "Gold Tower") is a dodecagonal military watchtower in Seville, southern Spain, built by the Almohad dynasty 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. This tower has a lesser-known half sister: the Torre de la Plata, an octagonal tower.
Military function 
It is one of two anchor points for a large chain that would have been able to block the river. The other anchor-point has since been demolished or disappeared, possibly from collapsing during the 1755 Lisbon Earthquake. The chain was used in the city's defense against the Castilian fleet under Ramón de Bonifaz in 1248 Reconquista. Bonifaz broke the river defenses and isolated Seville from Triana. The besieged Muslim city soon surrendered to the Christian forces.
Historical role 
At one point in the last two centuries the tower came under threat when the adjacent road needed widening, but faced by strong opposition from the locals the demolition idea was dropped. In 1868 it was put up for sale as scrap, but this too was vehemently rejected by the city's inhabitants.
Today the tower, having been restored, is a naval museum, containing engravings, letters, models, instruments, and historic documents. The museum outlines the naval history of Seville and the importance of its river.
Media related to Torre del Oro at Wikimedia Commons