Planned Capital Cities | Museum of the City has some excellent links to some of the delightfully disastrous planned capital cities.

The technology level of your city will have an impact on the city layout. A modern city will likely have relatively a hidden water supply and sewage systems combined with fairly prominent gardens/parks. Pre-industrial cities are more likely to have prominent aqueducts and likely open sewers with private gardens more likely than public. Pay special attention to the cultural heart of your city: Roman cities were built around the forum and its attendant temples/public buildings, while medieval cities wer emore likely built around a lord's castle and market.

An important part of any city will be the slums. Lots and lots of slums, with their fetid lifestyles and regular fires.