My Super Mario Bros 3 was a very difficult SMB level set, at least as hard as "the lost levels" if not harder. World 3-1 was particularly brutal.
organic io: Finished (World 6 final)
A-Lin: World 3 Castle
I played it again tonight to take screenshots and see if I could discover levels 5-3 and 5-4, however I still couldn't find them, I think they don't exist. It just goes from 5-2 to 6-final.
2-1 - This is the extra lives loop exploit :)
Castle 2, make sure you choose the right way to go :)
3-1 , probably the most difficult level in the game. Massive jumping puzzle at this point and beyond that is very challenging
Castle 3. Indeed very difficult. You have to stay on the floating platform and get through all the fireballs and turtles. Intense.
Castle 4. Lots of jumping on moving platforms
World 6 final. It's the last level. Challenging, but not as bad as 3-1.
Awesome! Tell me what you want to play next?