It was built in 1460 on the tower in the western corner of the medieval citadel, as a result of the request made by the procurator of Olivenza, in 1459 to King D. Afonso V, with the aim of installing a clock. In 1509 it appears in the Livro das Fortalezas by Duarte d'Armas.
No one knows the date that the first clock was installed there. The last one, of mechanical machinery, is currently on the ground floor of the Municipal Ethnographic Museum, after having deteriorated and being replaced by an electronic clock in 1986. In 1792 there is a Providence of Queen Maria for the construction of a clock, which is not this last one. Its manufacturer is identified by the letters PF, which refer to the French manufacturers Prost Frères, so the clock can be dated to the end of the 19th century.
It has a hexagonal floor plan, with no noble materials. There are eight openings at the top. A large hexagonal pinnacle crowned by a weather vane tops the tower.
It is divided into two levels, linked by openings. In the upper level is the housing for the clock machinery and the bells. The lower level, which is hollow, contains the space for the weights.