Sarge follows your mouse pointer, soldiers follow Sarge in single file.
Hold the mouse button down to fire weapons, Sarge will stand still while firing.
Switch weapons using the '1', '2' and '3' keys (or buttons on the side-panel)
Hold down SPACE or SHIFT to stay still and look and aim freely.
There are many different breeds of alien invaders. Some of them are nearly invulnerable to the standard assault rifle, so it's a good thing Sarge is also packing a bazooka! Press '1' for the Assault Rifle, and '2' for the Bazooka. Be careful with the Bazooka because the explosions can hurt your soldiers if they're standing too close!
During the 2nd level of the game you capture an Aliens Weapon Factory which allows you to use a basic alien Atomizer as a third weapon choice. The Atomizer supports multiple upgrades leading to extreme power in the end-game.
All weapons and game stats can be upgraded between levels from the Weapon Factory upgrade screens, by spending Crystals collected from crates and dropped by the aliens.