Hirokazu Yasuhara who was one of the original designers for Sonic 1 & 2 has recently been discussing a lot of early concept and development work which went into Sonic 1 & Sonic 2.
This includes several sketches of level concept art posted on twitter and more interestingly an entire presentation about early Sonic 1 & 2 development builds, these include details such as how the time travel aspect of Sonic 2 would work including a level map, alternative zone layouts and early concept names for zones.
My talk starts from 4:50. In Japanese though.. 😛
— Hirokazu Yasuhara (@Yasuharah) May 30, 2017
The presentation is in Japanese however an English version will also be uploaded at a later date.
We’ll let you know when that happens.
Source: Hirokazu Yasuhara Twitter