GemCraft: Chapter Zero - Gem of Eternity

This free, Flash-based tower defense game is a prequel to GemCraft: Chapter One - The Forgotten featuring upgraded graphics and interface.
The player must conquer map levels in his quest to obtain the Gem of Eternity. The main offensive weapon are the gems created by the player using mana reserve (replenished by killing monsters), they can be used traditionally to power towers (which launch energy bolts at monsters), charge special magical shrines or as bombs, to destroy monsters, their nests and harmful beacons.

What makes the game addictive is the overall complexity (gems have various effects depending on their color, combination or whether they're used in towers or traps), also present in the award system: numerous amulets can be found or given for achieving difficult feats. Each reward also counts toward total player points, which increase player's skills, boosting magical powers.