[1] The idea of a fortification on the river Tagus to protect the maritime access to Lisbon was first suggested by the architect Francisco de Holanda in the reign of King Sebastian I.

The king entrusted Manuel de Almada to erect the structure, with the idea that crossfire would be possible with the then primitive Fort of São Julião da Barra on the north bank of the river, thus protecting the access channels to Lisbon.

[2] The initial structure, which was built in 1570, was constructed of wood but due to the shifting sands was not practical.

[1] During the reign of King Philip I two different design proposals were made by the Italian architect Giovanni Vicenzo Casale; one for a star shape and the other for a circular fort.

[3] A large number of barges were constructed in order to move blocks of stone from the area near the Fort of São Julião da Barra out to the sandbank.