If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
In this game UFO's will appear and kidnap the citizens. You can use the mouse to control a cannon and click to shoot the UFO's. Sometimes a UFO's will carry items, you can shoot it to get the item to help you protect the citizens. If a UFO catches a citizen and leaves the screen, then you lose. As the game progresses the UFO's will move faster and the game will be more difficult.
This game is played by mouse only.