Use the arrow keys or the WASD keys to move. If you touch the enemies you'll die. Once you reach the fifth level, you'll get a sword. If you have the sword, then when you press the spacebar, the ninja will use his sword. If you reach the fifteenth level, then you win!
Thank you @VinCrafts for the idea on how to make the ninja. Thank you @JC_ProGold for the music. I broke the game :( Nope! Just checked, and it's not broken.
