This is a series, about how the Sonic Games intermix Storys, so let's start,

I think that when Sonic defeated Dark Gaia at the end of Unleashed, and he was running through Apotos, with Tails, they spotted, Dr. Eggman's Amazing Interstellar Amusement Park, my guess he was working on it after Sonic 1 took place, since it was said that it took him years to make, and they went to investigate the place, and when Colors ended they were back in Apotos, but that's what I think.