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.
A sample of typical combat zone  in the we have bastion of Santa Barbara, the eldest of the six available to the Castillo de San Fernando.
The presence of the adjacent hills bastion of Santa Barbara, and especially the large expanse of farmland located in front of him and perfectly suitable for digging trenches made ​​this the most dangerous front and therefore should be the best defended. So are the large size of this bastion of hornabeque well as San Zeno, in front of him and also the largest of the Fort. For these reasons more works to give greater defense was built. This is the unfinished gentleman located inside the bastion of Santa Barbara.