If you just want to play a similar city-builder from that era, try SimCity 4 Deluxe (Steam/GOG) or Cities: Skylines (modern alternative).
If you've dusted off an old physical copy, the serial code is typically found in one of three places:
City Life is a life simulation video game developed by Monte Cristo Entertainment and published by Codemasters. The game was initially released in 2006 for Microsoft Windows and later received a special edition, City Life Edition 2008, in 2007. This special edition came with new features, items, and gameplay mechanics that enhanced the overall gaming experience.
Upon entering the code, the game’s installer would run a mathematical calculation to ensure the string of characters followed a specific algorithm designed by the developers.