Hold the arena against eight waves of enemy soldiers. Enemies shoot back, rushers run at you, and heavy gunners take a lot of hits.
On a phone: drag with your left thumb to move. Touch the right side to shoot: it aims at the nearest enemy automatically, or drag with your right thumb to aim yourself.
On a computer: W A S D to move, aim with the mouse and hold the button to fire (or hold space to auto-aim).
Defeated enemies sometimes drop a shotgun, a rifle or a health pack. Walk over them to pick them up, and use the grey cover walls to block bullets.