GemCraft

Havoc and corruption swarms through the land, and you are one of those few wizards who can put an end to it. Create and combine magic gems, put them into your towers and banish the monsters back to hell!

the basics:
click on spell buttons to select them
drag gems to move, throw or combine them

advanced:
1-6 or B/T/C/M/W/G selects spells
hold ctrl to create/combine as much gems as possible
hold shift to place multiple towers/trenches
hold shift to create multiple gems
P to pause
click/space/enter to close tutor panels