Summary
Learn the way of the ninja, and enjoy while at it.
The Good
Ninjas, acrobatics, sailors, robots, magicians, tanks, mechanic snakes etc. etc. Whats not to like.
Everything works well in this game. Your ninja does exactly what he/she is suppose to do. Enemies are challenging, but never cheat (as there is always a way to get away with no hits). Boss fights are intensive, and level of difficulty increases smoothly.
Also weapons are exactly what you would expect from a ninja. sword, chainsickle, different Shurikens (advances with primary weapons) and bomb. Bombs are most boring ones, but usually serve their purpose in places they are found in. Sickle and sword are primary weapons, which you can choose from time to time, and they both are equally good, which is another sign of well designed game (actually, after sword gains distance, it becomes the weapon of choice in boss fights, as it's much faster). Even though some parts give you hard time, the game is simply enjoyable from the beginning to the end.
The Bad
I liked just about everything in this game. But if I have to dig up something to complain about, I would say that ninjas in the future, is not the most creative plot. Also, since it is old NES game, there's not much replay value (although it's fun to play through couple of times).
The Bottom Line
If you like ninjas, sidescrolling action and intensive gameplay, you will love this game.
