Video game levels are designed with one thing in mind: fun. But that fun comes dressed as architecture. Cathedrals, castles, office buildings, homes. The kind of stuff architects are best at.
The Architect's Journal have posted a list of what they feel are the top 10 examples of architecture in video games. The list itself is partly tongue-in-cheek, so we can forgive its alarming lack of knowledge of gaming environments (no Ico?), but its point is not to authoritatively catalogue the best buildings.
The point is more likely to simply get you thinking about architecture. To stop thinking of the environment in a game as a level, and appreciate it as a building. Its design, the materials used in its construction, that sort of thing.
May sound a bit naff to many of you, but you want games to be considered art, this is part of the deal. Top 10: The architecture of computer games (part I) [AJ, thanks Greg!]