WASD to move. Left-click to fire in the mouse's direction. Q to change ship sprite. E to change missile sprite. SPACE to toggle shield.
3 levels. Each level increases the number and speed of invaders, the number of missiles, the background music, and the sound effects. Once at level 3, the invaders will keep spawning until death.
Player has a total of 3 lives. Once all lives are exhausted, the game is over.
Restart the game to play again.