Take a role of an evil overlord of orcs, undead and demons and protect you gems from being stolen by good heroes.
Try to annihilate the enemies before they grab a gem, but even if they do, you’ll have a chance to annihilate them on their way back and you’ll save the gem.
Use mouse (hold SHIFT to build multiple towers or cast multiple spells). You can also use hotkeys (see built-in hints for buttons etc.)
The game has a bult-in tutorial.