Or, you could draw 25 individual buildings, scan them, then use 20 duplicates and scatter them about throughout your city. With that many original buildings, the copies wouldn't look so obvious. And you can slightly resize the buildings so you have larger and smaller versions and not obviously copies of others. Then make the specific unique structures: castle, 3 temples, 1 shrine/park, docks, walled sections.

Though still some work, its not like drawing 600+ buildings from scratch.

Just how I'd do it.

GP