Released in Japan in 1988, Ninja Gaiden soon became an essential game to own when it debuted 1989 in USA. Quickly developing a reputation for it's fast game play and difficulty it soon gained the respect of gamers and still known as one of the hardest games in the NES library. Difficulty only upped by the fact that enemies respawn if you are in the wrong position to move forward, go backward in the level or in a standing spot where enemies begin to spawn, and respawn. This problem is infamous with many enemies but most hated are the birds that come very soon after the beginning levels of the game. The best way to avoid this problem is to move forward, use your ninja techniques properly, and know your boss fight gimmicks that'll lead to their easier defeat. Soon you'll be knocking on the end game levels and will experience what is the most unforgiving levels in the game.