Use the mouse to create and position defense units. Hotkeys: B - Barracks, F - Factory, C - Air Command, X - Mine, T - Trooper, G - Grenadier, K - Marksman, E - Elite Trooper, N - Machine Gun Turret, A - Artillery Cannon, R - Grenade Launcher, L - Laser Cannon, M - Missile Launcher, S - Plasma Cannon, D - Disruptor, I - Ion Cannon, and 1-4 - Air Strikes.