CRISIS CITY Restored!
Restored the CRISIS CITY Stage Gate.
Story related, can't be missed. This will unlock after you finish both Acts 1 and 2 of Crisis City. You will also set Blaze free.
WARNING: Many of the achievements are glitched in Sonic Generations. If it doesn't pop for you when it should, you may have to get it again on a new save file on a new save device like a flash drive or something. This is the only way I've heard of. And don't complain about it here, I had it happen to me with the achievement for collecting EVERYTHING.