The game is based on Augmented Reality, which means that you see your own world on your screen, and play the game in there. The game starts by instructing you in finding a flat enough surface where the game can be placed. Remember also to walk around the levels to find hidden structures and best angles for shots!
A good area to play allows for some space around the game. If your space is limited you can also turn the level around with the rotation buttons on each side of the level.