Sonic the Hedgehog 1 for SegaCD is a ROM hack or fan-made adaptation of the original Sonic the Hedgehog game, ported or adapted to the Sega Mega-CD. It's likely based on the original Genesis/Mega Drive version, but with potential enhancements or changes to utilize the Mega-CD's capabilities. Gameplay would involve the classic Sonic platforming experience of fast-paced running, loop-de-loops, and defeating enemies.

Sonic the Hedgehog 1 for SegaCD

Sonic CD with the Original Jump Sound