Your mission is simple, stop the enemy move from the start point to the end point. Everytime one escapes, you lose a life.
As enemies are killed you will earn more cash to spend on further towers and upgrades. Purchase the various towers from the menu at the bottom and place around the battlefield to build your defenses.
Build more complex defenses forces the enemy to travel further increasing your chances of dealing more damage.
Unlike most enemies, aircraft are not force to find a way round your defenses and will fly directly towards their goal.
Some towers have additional effects like slowing the enemy down or setting them on fire. Burning enemies will move faster.
Remember to make the most out of money and space by upgrading your towers. Each tower has thress levels.