The objective is to protect the Princess by defending her tower from the relentless onslaught of enemies. To do this you must pick the enemy soldiers up and throw them off either side of the screen. Simply use your Mouse to drag, and release them again to send them flying.
The tower can only withstand a certain amount of damage.
You must keep the tower standing for as long as possible. You also have special powers which can get you out of nasty situations and deal out massive amounts of damage.
To activate a power simply select the corresponding icon located at the top left corner of the game screen.