Prevent Chaos'' troops from reaching the crystal of time at the end of the path. You will lose life/energy for every enemy that slips by.You will get money each time an enemy is killed. Spend your money to buy new towers, to upgrade the existing ones, and even to move to the next age. Each enemy has is own characteristics (armour, flying, speed, etc.) and the tower you''ll choose to create will need to take it into account.