Tallinn seems to have been built of grey limestone, paekivi, they call it here. It is not a very dense stone (not too hot on geologiy, but I think it's quite porous and, with Estonian wetness and winters, suffers from freeze-fracture) and many old buildings have crumbling walls. Obviously too, perhaps, it depends on age. This one probably dates back about 100 years, and so is in reasonably good repair. I like the marrying of modern around the old. Or, rather, the rather boring modern structure is redeemed by the old.