You can choose from a number of different elements to build towers from.

Four Second Fury

Belter
A game based on the popular "Warcraft 3: Element Tower Defense" and "Starcraft: Turret Defense" gameplay. The object of the game is to prevent the hordes of monsters from getting to the castle by building element towers near the path.
Use your mouse.

Aqua Energizer Most people should be familiar with this kind of puzzle game. Go through levels by finding the way for your aqua-man to get to the exit.