You can't actually fit towers or anti-AA in these, only anti-tank guns or sniper towers. (or, less ideally, houses/oil buildings). It fits 2x2 tile buildings.
They are, however, by no means useless. Maxing 6 of these will grant a huge defensive asset to your base, basically like 6 more mini Versailles (minus the HP boost). It might work only on a smaller area but it is much more slowing down. Even lvl 1 slows down more (52%) than Versailles. Plus, they are additional walls.