I wanted a hack to only use World 1. The castle in World 1 would have the final boss. Would it be as simple as just placing everything in the castle, or would I have to use ASM hacking to get it to work?

I think you can do it with worldmap editing. Not sure though.
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Maybe you can use this editor to remove the buttons from the bottom screen. (Idk because I've never used it myself.)

Then, just do some ASM/worldmap editing to prevent the game from advancing to world 2.