User blog comment:Smash The Echidna/A curious observation of mine/@comment-32712012-20171222081009/@comment-32712012-20171222095303

I'm not sure quite what you mean by "what city is that". I provided a link, but I don't know if I can get much more specific than that.

As to where the city is, I get the impression that it's right next to Green Hill Zone. (The intro Sonic Forces cutscene seems to depict Sonic rushing into this city right after going through Green Hill Zone, which is what leads me to this belief.)