Gemcraft is a fantasy themed tower defense game where the object is to destroy all the monsters before you run out of mana. Mana is gained over time and by destroying monsters. A monster who makes it past all of your towers is banished for a large sum of mana. If you cannot afford to banish the monster you lose.
Defending is done by building towers and creating gems which cost increasing amounts of mana. Gems come in 8 colors each doing damage and a secondary effect to the monsters it strikes. Secondary effects include Mana gain, poison, stun, slowing, and several others. Gems can be combined to create higher rank gems which deal more damage. Gems can also be thrown as bombs to do area of effect damage.
After each level is completed experience is awarded which allows the player to learn skills. Skills were able to make gems and towers cost less mana, making gems more powerful, or granting starting gems. Old levels can be replayed with higher skills to gain more experience, allowing the harder levels to be completed.
One of the most popular and highly rated games on Kongregate.