Oh that reminds me of some numbers:
Zero-K has 16 defensive structures. This is not counting jammer, anti, behe and superweapons.
-> ZK has more defensive structures as other games have buildings per faction. (sc(2) anyone?)
5 of these structures can
only target air units.
-> One of ZKs earliest design goals was to get rid of artificial target limitations.
ZK has a total of 16 units (mobile+static) that can only target air units.
-> There's 19 buildable air units. Not counting drones. Soon we will have more AA units than air units.