GemCraft is a series of fantasy themed tower defense games where players create fortifications and the gems that power them. GemCraft ties together the player's health and money, every monster not killed reduces this total, as does casting spells or creating gems or towers. The series allows players to replay stages for more experience, and can take a very long time to fully clear.
The series started with GemCraft released on June 26, 2008. The series includes GemCraft Chapter Zero released on April 16, 2009 and GemCraft The Lost Chapter, released on Feb 17, 2011.
All of these games can be played for free online, although the most recent game has a variety of premium content which must be purchased to be used.