Given its privileged position, located on a hill in the mountains of the Serra Grossa and on the left bank of the Sénia river, Ulldecona is the perfect example of a frontier castle. During the Arab occupation, between the 8th and 11th centuries, it was an Andalusian fortification. With the conquest of the lands south of the Ebro by the Christians, it passed to the Montcada family of Tortosa in 1148, who gave it over to the Order of Knights of the Hospital of Saint John of Jerusalem, and it was transformed into a Christian Castle. A village grew up around it, thus being protected from Muslim attacks.

Currently, the fortified complex consists of three buildings - two towers and the old church - and remains of other demolished buildings, all surrounded by a perimeter wall. One of the most emblematic structures is the circular tower from the 12th century, which served as a watchtower. The upper terrace of this latter building, of which the battlements and embrasure have been preserved, offers unparallel views over the surrounding area. It is complemented by the square Keep Tower from the 13th century, which housed the stately rooms. 

The Church is the most modern building within the enclosure, dating back to the 16th century. It may have replaced a primitive chapel located in the main tower of the Castle.

Although most visible structures relate to the Christian military fortification, the remains of the first Arabic Castle, such as the remnants of the wall topped by two watch towers, are still preserved today. Also within the enclosure, there are the walls of medieval dwellings and even the remains of an ancient Iberian settlement.