A horde of horrible monsters is attacking your kingdom. In Tower Defense, the only way to stop them is to build your towers fast and survive every wave.
Enemies march along a set route towards your castle. Build towers in the best spots along the path, and the soldiers on them attack as monsters pass by. Every enemy you defeat earns gold. Use it to build more towers and upgrade the ones you have, because the enemies get stronger each level and basic towers will not be enough. If arrows and stones are not doing the job, use the power of the elements. If your castle is destroyed, you replay the level.