GemCraft Chapter Zero: Gem of Eternity is a prequel to GemCraft and is very similar to the original. Both are fantasy themed tower defenses where monsters attempt to get by your towers to attack your base. You win the level by destroying all the monsters before you run out of mana. Mana is gained by destroying monsters and lost by a monster hitting the base or using abilities.
You defend your base by building towers and traps and filling them with gems. Gems can be combined to make stronger ones, dropped on monsters to do area of effect damage, or dropped on shrines to have helpful effects. Monsters now have some buildings to aid them as well; these can be destroyed with Gem Bombs.
Completing levels earns experience which allows you gain powerful skills which you will need to complete future levels. There are several achievements (amulets) that boost your experience gain if you earn them.
Old levels are available to replay for more experience, this time in ten different modes, with the top score from each counting towards your experience total. In other words, there is a huge amount of game-play here.
This is another highly played and rated game on Kongregate.