The Torre Velha Fortress is one of the most important examples of Portuguese Renaissance military architecture, having been built by D. João II, in place of the former Forte da Caparica, built in the reign of D. João I.
It was one of the first integrated artillery systems for the defense of the Tagus estuary, along with the Santo António de Cascais Tower (Cascais bastion) and the São Vicente de Belém Tower. Later, in 1571, D. Sebastião enlarged the construction, transforming it into the Fortress of São Sebastião da Caparica. During the Philippine occupation it underwent some changes in the structure and became known as the Castilian Tower. At the end of the 17th century the fortress once again received consolidation works.
Deactivated in 1801, the fortress began to function as a lazareto, welcoming boat travelers who needed to be quarantined after landing. It was reused for military purposes in 1832, but a few decades later it was only used as a warehouse and accommodation.
At present, the fundamental parts of the mid-17th century remain, as evidenced by a plan designed in 1692. The plan develops in a U shape, consisting of three bodies and three bastions with barracks. One end of the fort is extended by a stronghold and the watchtower. Next to the door of arms was built the chapel, dedicated to St. Sebastian. The central body of the Old Tower is of quadrangular plan, lowered, to which the governor's house was attached. The old gate of the square, next to the tower, bears shield with the arms of Portugal.