Those houses are beautiful aren't they?
Mordheim is a city that has been hit by a comet. We have discussions on the forum occasionally about intact non-ruined buildings surviving in the city. Given medieval construction, I would tend to doubt it, but if it makes your games more interesting, why not?
As far as Mordheim 'The Game' goes, you can elect to play in a non-ruined setting such as Mutiny in Marienburg which would encourage an intact living city rather than a ruined, deserted one. My group spends quite a bit of time playing in the Mordheim equivalent of Mediterranean, North African or Caribbean towns so most of our buildings are of a style likely to be encountered in that part of the world and are designed so that the interiors are accessible and playable. It gets a bit fiddly now and again, but mostly its just too cool.
For examples follow the link in my signature.