Enemies will try to attack your cave, stealing your gems. Prevent them from taking them out of the game (if you lose all of your gems, the battle is over). Buy towers to place along the enemies' path. Earn money by killing more enemies.
When a tower has killed enough enemies (gathered enough experience points) to level up, you'll see "UP" above the tower. Click on that tower to open the upgrade window.
The Den can be placed on grass tiles.
The Crypt can be placed on snow tiles.
The Temple can be placed on rock tiles.
No towers can be placed on forest tiles.