In this game well you have to protect the precious stones of good characters. You play for the side of evil and you've got the forces of demons and orcs. Excellent game tower defence genre.

Use mouse (hold SHIFT to build multiple towers or cast multiple spells).
The game has a bult-in tutorial.
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.