Cities can have a general style whereas city-districts and quarters can have very distinctive styles,too.
So you could describe a city as being a mix with predominantly X-architectural style.
Most buildings and infrastructure in a city will have been build in a certain time span depending on its place in history. This time span defines the style then.
I studied urban development a little, and they talked about a city's style often and by saying style you're actually saying "architectural style" but omitting "architectural".
Style mostly revolves around visible trends and if you apply this to a city, it must be about its buildings and thus architecture.