After looking at wikipedia entries for castles and fortifications, I can see that there are many technical historical-architectural terms which are very precise and detailed.
The encompassing term might be fortifications. This is technically probably a bit wide as it would include military constructs which people do not inhabit and might (technically) miss some of the buildings you are trying to describe. (You might go with fortified historical residences or some such term)
On the other hand, depending on technical correctness you are aiming for, pragmatically and simply castles might be a good tag (you can explain in the tags summary that it also includes citadels, stately homes, etc.. )
Both the term castle and fortification are taken from this paragraph in wikipedia:
Castle is sometimes used as a catch-all term for all kinds of fortifications, and as a result has been misapplied in the technical sense. An example of this is Maiden Castle which, despite the name, is an Iron Age hill fort, which had a very different origin and purpose. Although "castle" has not become, like chateau in French and schloss in German, a generic term for an English country house, many of these use the word in their name while having few if any of the architectural characteristics...
Combining common characteristics of the terms that you want to describe
- Historical private residences (is it really necessary to say private, I wonder)
- Historical or fortified residences
Finally, I am not a historian nor an architect, so maybe asking in a more specialized environment might yield technically more appropriate result (wikipedia discussion page?).