After you beat the robot bosses, you have to fight through half the levels again. And the levels are badly damaged, they have a lot of pit falls, and you have to fight the bosses from Mega Man 2 again!
Then you go through Dr. Wily's castle, for apparently the last time. Yeah, right. I liked the ending to this game. But I guess more sequels had to be made.
This game, Contra, and Bionic Commando were the hardest NES games I remember beating without cheating.