Far from a fan game. Taxman created his own engine for this and has redrawn and created his own sprites and animations. There is some cool stuff in this game and it's the game Sonic 4 should have been. Look at the elevator panel...
M. Sonic Mania
K. Sonic and Knuckles
3. Sonic the Hedgehog 3
2. Sonic the Hedgehog 2
1. Sonic the Hedgehog 1