Shoot down the spiders and complete all sixteen levels. Collect your ammo from the supplies board on the left side of the screen.
The fly in the top left side of the screen gives you a colour to place on the grid below. Left click a block to change its c
Place towers on the field to prevent the monsters from reaching the opposite end. Move your mouse over one of the towers to view the cost and details. Select the tower you want to build and then click on a square to place it. There is a limit to how many