I'd say "defensive structure" is a perfectly fine term in general. If you want to vary it up, "fortification" can be used when something has some extra stuff added to it to make it more defensive, such as in the case of walls and towers. "Defensive system" is another good option, as it takes into account the idea that all the different structures are working together toward a common goal.

But no, to my knowledge there isn't any special term for such things, and "defensive structures" or "defensive system" will do just fine.