The keep, 37 m high, stands out from the rest of the fortress, being the highest of the fortress towers in the Spanish-Portuguese border. Built in masonry reinforced by stone ashlars at the corners, it has 24 arrow slits that illuminate its interior. At the top of its four faces, you can still see the remains of the primitive machicolation that defended its flanks, one of which was rebuilt in reinforced concrete around 1973. At the foot of the barbican, a moat was dug, ordered by D. João II in 1488, completely surrounding the fortress.
John II of Portugal, the Perfect Prince, ordered the keep to be reinforced and made it the hightest among all the fortress towers in the Spanish-Portuguese border.