Long before Retro City Rampage, developer Brian Provinciano was working on Grand Theftendo, essentially a demake of Grand Theft Auto III. Brian even created his own original Nintendo development kit to make this happen! He would eventually move away from making a little demake stuck within constraints of old hardware, with the result being Retro City Rampage.
With the release of Retro City Rampage tomorrow on WiiWare, Brian is including a special homage to his original concept by including a truly 8-bit version of the game called ROM City Rampage. This additional content will also be released as a free update for the Steam version tomorrow.
It's not a simple process demaking a game, and this video from Brian shows you how he managed to pull it off.