If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Get out of the classroom and get some practical experience in all-important space defense.
Place towers and turrets with your mouse to protect your Core Generator from the invaders. There are four different tower bases that can be purchased over time, each capable of holding one of three specialized types of turret, each aimed at destroying a certain type of enemy. Click on the towers to open the tower menu on the right and click the plus signs to upgrade Damage or Range, or sell or power down towers when you need to.