GemCraft Tower Defence

Similar to Desktop Tower Defence this game is based around defending against waves of enemy minions. In this game every tower can be made into a weapon by placing a gem on top of it. The various colours of gems have different powers, and you can combine them to make gems with mixed colours. The gems also have different strengths depending on the shape - by saving up you can purchase more powerful gems.
There are lots of levels, and the aim is to complete them all above a certain level, which I estimate would take days. Luckily you can save your progress and come back to it later.