You can add AC systems in the attics and then run the piping inside the building. You keep the facade as it is.
I guess it depends on the country, customs and other things. In my part of Europe, older people are convinced the AC will kill them if they use it. So YMMV.
that's not very efficient since attic will be then hot
balcony is more practical place where to hide it if it's historical building, worth that <0.5m2 if you don't hang it on wall and must hide it from neighbors/bureaucrats
You can indeed. But then you lose significant space internally and not to mention the noise. Of course, better than having nothing. However, space in these houses comes at a premium as well.
I guess it depends on the country, customs and other things. In my part of Europe, older people are convinced the AC will kill them if they use it. So YMMV.