It is part of the rampart fortification, built in the 17th century for the Wars of Restoration. This wall had three gates, of which only the most monumental, the Calvary gate, facing Portugal, has been preserved.
Surprising is its production of rusticated and bush-hammered marble ashlars. The door opens with a voussoired semi-circular arch, separated by two small stones that penetrate the frame of the door. A small cornice line leads to the uppermost part with a triangular pediment that is split by the openings of the drawbridge mechanism. In the centre of the pediment a rectangular motif includes, in unhammered marble, the royal crown on the coat of arms of Portugal. Below this is a reproduction of a Calvary, to remind us that the construction of this wall and the gate meant that the Calvary previously found here had to be removed. Under the Calvary, a Latin inscription recalls that: "From the maintenance of faith and religion comes the expansion of the Empire". A last line includes the date 1703.
On the sides of the pediment, the acroteria are topped with spherical motifs.
Inside the tunnel, at mid-height and on both sides, there are windows framed in marble, where the guards were stationed. Near the door, behind the walls, there is still an armoury.