Ok so first off, this is a really nice game. The controls are smooth, the graphics are neat and I love the simplicity of it. That being said, I do have a few criticisms.
The voice acting is a little off... it seems like you had the direction down, but should have gotten someone else to do it rather than doing it yourself.
Most of the time the ninjas are just aimlessly running around not attacking you, I understand that this is likely by design, the AI could be tightened up a bit.
The random highspeed dart that flies out of nowhere is something that IMO, should be introduced a bit later in the game after some scaling.
I hope you don't take any of these criticisms too harshly though, because I really dig the game. Look forward to seeing more of your work!