Taken it down 3 times.
Near the End Started crapping me off a lot.
Had barely any items left..
So i had to salvage arrows from those goblin things that shoot you.
10 - Supply Depot (1)
13 - Barracks
15 - Marine (1)
16 - Orbital Command + Supply Depot (2)
16 @100% Marine (1) - Command Center (2)
21 - Bunker (1) + Barracks (2) (3)