The fort, which is relatively small, was built on a rocky outcrop into the Tagus estuary in a Mannerist style. It has a pentagonal outline, with the roof covered by a terrace, with two circular bartizans. There are six gaps in the walls for gun emplacements. Using the site of an earlier artillery battery, the Fort of Giribita was rebuilt and enlarged as a result of a decision of the Council of War created by King John IV. Work was supervised by António Luís de Meneses, 1st Marquis of Marialva and was completed in 1649, according to the inscription on the entrance. By 1735 a report noted that the fort had just two pieces of artillery, with a further five not fit for service. In 1763 it was reportedly armed with 7 cannon but by 1824 the functioning artillery was again back to just two pieces. During the Portuguese Civil War (1828-1834) the Fort of Giribita was armed with 22 gunners and seven cannon.