Good but too slow
First, let me say that the artwork in this cartoon is beautiful, and it was actually fairly good, and I'm surprised that you managed to do something with a length this long (very ambitious)
However, the actual animation, acting, and plot were all very limited. I wouldn't think a ninja would want to kill every single guard without a good reason (it should be a fast in/out job without a disturbance, right?). Also, the animation lacked any fluidness or any sense of weight.
My biggest gripe with the cartoon is the timing. Some sequences were too slow or held on for too long, and the cutting of the cartoon is really too choppy. The whole piece just doesn't flow naturally.
Don't get me wrong, it's still good, but I'd "study" other professional cartoons (stick to movies or anything done from 1940-1960 not on television. Television cartoons like this also tend to be drawn out waay too long, like Dragonball Z). Also, go on YouTube and watch videos where people do flips and jumps to get a sense of how humanoid creatures should move