BEST WAY TO BEAT NINJAS: build a temple or other tower as close to the entrance as possible. Hits ninja, activates stealth, wears off by the time he gets to your towers
I am sorry but i really cannot like this game. This game manages to be really frustrating and the acceleration + floating + wall jumping all behave in counter intuitive ways. This is not cause I'm a noob. I have beaten JUMPER 2 which is much more challenging while being less frustrating. If you like this game I would really recommend going and playing that one.