What I'm planning to do, once I get around to building the city wall I've been wanting for ages, is to make sure I have some scatter terrain with it so I can also use it in modern settings.
Think a medieval town gate with modern traffic signs and a car or two; all of a sudden it's part of 'the old town' in an ultra modern skirmish. Replace the signs with some halberd wielding guards, and it's back to being a medieval or fantasy town gate...
The same will go for the buildings inside the town walls too.
And the estimate of 18th century buildings still existing is a very conservative one. In my city alone, we have loads of examples of buildings going back to the 15th or even 14th century (plus older ruins). And I know of (and visited) many a place in Europe that has buildings dating back even further.
Basically any 11th century and younger building could possibly be used in a European setting. And if you include aquaducts, arenas and monuments, we could actually look back even further, all the way to Early Imperial Roman times!
So have at it; context is everything in this case. Adding a couple of pieces of time specific scatter terrain, and no older building should look out of place in your games...
