Use different types of towers with different ranges to build a maze. The longer the creeps need to reach the middle, the longer you can shoot at them. Survive the waves of creeps and progress through the levels - with each level your platform will grow bigger and will spawn more creeps, which get more powerful as well.
Tower Defense
This game only features an Endless Mode - your goal is to hold out as long as you can.
There is different types of Defense Towers and each of them (once placed) can be upgraded to a certain level to get stronger - how long can you survive?
Want more?
Let me know in the forums - if you generally like the idea but want more features or challenges, let's discuss it in the forums. If enough people make feature-requests I'm going to make a sequel.