We've added turrets in version 0.3.4 of Avoyd, a new tool "Max" in the voxel editor, and fixed several issues - details in the 0.3.4 change log.
You can spawn up to four turrets, with a cooldown between spawns. Once you've reached the max number of turrets, the cooldown lengthens and the next spawn replaces the first turret you've deployed and so-on.
Video showing us using turrets in a coop LAN game:
We're working on in-game building.