Is this photo an actual set ? If yes which one, and if not, any idea what it's based upon ? (if it's not a canon set, my own research suggest that's it's made from 6085 but I'm not sure).
I have identified a unique part, to help with search - Panel 2 x 5 x 6 Wall with Window with Window Stones Light Gray Pattern which has been used here.
It has been used in just two sets listed seen below, none of which look like your model
Black Monarch's Castle (6085) from 1988:
Black Knight's Castle (6086) from 1992:
So it seems like someone made a great MOC back in the days!
This is pretty clearly a MOC created by combining elements from multiple Castle sets. None of the castle sets were nearly this large for one thing. The only pieces that are distinctive are the shields and the black castle window panel pieces which are from the Black Knights sub-theme. From the size of it, I'd say this is 3 or 4 castles put together.