That's actually a good question! At the moment, they're AI controlled, I'd rather not have players have to constantly man defences if I can help it, as it's not the most fun thing in the world. However, I am considering some defences that are unmanned that a player can jump into as well. Those would be smaller affairs, more a gun on a mounting than a full position like these. Having a few positions where the defences can be bolstered if needs be is a fun idea. (They could even be purchasable and deployable/undeployable... Plenty of ideas for things that could be done, but ATM I'm trying to keep it simple with an anti-infantry, an anti-vehicle and an anti-air defence for each side.)
As an addendum, I've got ideas for larger defences, for larger, higher tech bases (stuff like artillery guns, full sized SAM defences, CIWS type things...)