Robot armies are attacking your space headquarters. Stop them whatever it costs. Protect the hq gate by gate installing turrets, catapults and barricades in their way. Use your resources cleverly, or you'll run out of your budget fast.
Two difficulty modes: normal and hard with online hiscores.
S - send next wave
P - pause
M - mute/unmute
ESC - discard/unselect current item
F - toggle fast-forward mode
1,2,3,4,5 - upgrade propery when item selected
R,T - downgrade range and spread (catapults only)