Instruction: Waves of zombies attack our hero.
At the end of every wave:
The hero can buy gun updates.
When a zombie hits the base, hero's health gets reduced by a certain percentage.
When the health becomes zero the game ends.
Drag the mouse to move the cursor.
Keep the mouse pressed to shoot continuously.
Release the mouse to stop shooting.
Beware of the skeleton zombie. If it hits the hero the game ends