Programming all the copy abilities had to be a nightmare. Not only the graphics but the controls for things like the wheel & hi jump, the pallet swaps for the Freeze abilities, the environment interactions from the Hammer... it's a ridiculous amount of content by today's standards and it was made over 30 years ago.

Then add in cutscenes (all in-game engine, but still), between level overworld sections, mini-games... It's baffling!

Half of that game would be DLC/premium content if it was made today.