It wasn't so much Wily's Castle that was hard, but Wily himself. That last form with those homing energy balls is just ... wow. I don't know how you're supposed to beat that.
It took me several tries, and it's a beast, but it's not bad once you figure it out. It's been awhile, but I remember that I used the galaxy power to suck up his attacks and maybe two other powers for the actual attacks. I was also pretty well stocked on E tanks.
Yeah, but E tanks are so cheap. I want to beat Wily with just my health bar and preferably just the Mega Buster too. None of his attacks are random, so it is technically possible. It's on my list of things to do right behind beating Battletoads.