fortification
n//ˌfɔː(ɹ)tɪfəˈkeɪʃən//befästning, befästningskonst, fortifikation
The act of fortifying; the art or science of fortifying places to strengthen defence against an enemy.
That which fortifies; especially, a work or works erected to defend a place against attack; a fortified place; a fortress; a fort; a castle.