It's nice to play a defense game that actually makes an effort to incorporate a story that unfolds as the game progresses, as this one does in short scenes between battles or dialogue before and after. We're not talking Tolkien or Jordan here, as far as fantasy stories go, but it serves as a nice incentive to keep playing.
During battle, click on the prince and drag the cursor to pull back the string on your bow, which determines how far your arrow will travel. Release the mouse button to let the arrow fly. Destroying enemies gets you points, which you can use to purchase various defensive and offensive upgrades between battles. You can click on your castle at any time during battle to change the types of arrows you fire, once you've acquired new ones, and to use your upgrades.
Despite some control and initial difficulty issues, Arrow of Time is an impressive example of a game that manages to be both action-packed and cinematic. Taken with a grain of salt and some patience, it can easily win you over if you give it a chance.