Sonic The Hedgehog 1 for Sega Master System editor by WV, extended by Maxim

This is a ROM editor that lets you edit various aspects of the game.

Currently editable:

• Level layout
• Tilesets (load and save images and edit in an image editor)
• Object positions and types
• Level flags
• Level blocks (32x32 blocks making up the levels)
• Level names
• Static screens
• Palettes
• Tile sets
• Sprites

Still to come (in no particular order):

• Lots more UI improvements to make editing easier
• Fully removing size restrictions (currently sticking to the original game's data arenas for some parts)
• Music/SFX editing/replacement
• Adding a level select, sound test, game saves?
• Ability to add any number of levels, tilesets, etc
• Sonic 1 GG support

Downloads in the Releases section.

Feedback and discussion is welcome at this SMS Power! forum thread or you may raise issues here on GitHub.