In "item" screen in Renegade X you can currently buy Nuke/Ion and airstrike.
In video there are more options that are grayed out.
Bottom two are two defense turrets.
I imagine that those turrets would, when purchased, go on their appropriate places
(like those empty metal things in middle and in front of base on map Walls)
Could probably be re-purchased when destroyed.
That is in some aspect, building base defenses, but main buildings remain as they are, destroyable-only.