"A playable world map that includes stage-like elements ... But how do we evolve a level-based platformer like Sonic into this new Open Zone? That's what Sonic Frontiers is all about," said ...

The Hidden Palace Zone was intended to be a bonus level for players after Sonic had collected all of the game's Chaos Emeralds. But the map was cut during development and left to lie half-finished ...

To unlock a bonus level that becomes available on Sonic's map, beat the game and collect all Chaos Emeralds with Amy, Cream, Sonic, Tails and Knuckles.

You basically need to keep dropping as far down the map as you can (falling down ... to skip ahead to it next time, using the level select menu in Sonic 2. When it says 'press button' on the ...

YouTube user Beatz rebuilt the Windmill Isle map from Sonic Unleashed ... The game has been well-reviewed by Sonic fans, and this level rebuild is a great addition to a great passion project.