A bastion is an angular structure projecting outward from the curtain wall of an artillery fortification. The fully developed bastion consists of two faces and two flanks with fire from the flanks being able to protect the curtain wall and also the adjacent bastions.  Bastion fortifications offered a greater degree of passive resistance and more scope for ranged defense in the age of gunpowder artillery compared with the medieval fortifications they replaced.
The Santiago, situated to the northeast, and San Narciso, Southeast are equal to each other, are connected with the square and inside a store of gunpowder bombproof, whose vault is arched and has  buttresses interior.