Airfields are protected by two(1) fast firing ack battery, usually located between the runways. They fire up to 5k.
Base Power Stations- These provide power to the base. I believe, but have yet to prove, that if these are damaged, it takes longer to rebuild damaged bases (or closed bases after capture).
 
 
Hangers- These provide maintenance to planes flying from that field. If they are damaged, then planes will be easier to kill, and may leak fuel or oil even before being hit.
 
Tower- this facility provides radar coverage in an area 20 miles around the airfield
 
Fuel Cells- These supply fuel for the aircraft flying from this field. If they are damaged then planes will suffer from reduced performance due to a lower octane fuel (note, the 109 is supposed to be imune from this effect)
 
Ammo bunkers- these supply ammo to the planes flying from that field. If they are damaged, then planes flying from that field will do so with rediced ammo loads, more than likely without shells for their cannon.
 
The Herd- The local herd of sheep, a source of pride to the pilots :-) Note Ammo Bunkers in the bacckground