Waves of attacking enemies are to be destroyed by our Hero.
At the end of every wave:
The Hero can buy gun updates having lesser bullet lag times, which also include health updates.
He can also buy updates for lesser % health loss. This applies to soldiers only and not to other enemies.
When a bullet from an enemy hits the fort the health gets reduced by a certain percentage. When the health becomes 0 the game ends.
Keep the mouse pressed to shoot continuously. Release the mouse, and drag the cursor to rotate the gun and change the direction of shooting.