The native English speakers on the team advised that "Whose is that castle." is not good standard English and it is better if it is not accepted. I cannot comment on it myself.
Please also note than in other sentences we do distinguish and we will distinguish:
This is an old castle.
This castle is old.
as two different sentences, not mutually acceptable.
It's a minor difference because of the word order, but the difference between "Whose castle is that?" and "Whose is that castle?" would stress the importance of "Whose" in the first case, and "that castle" in the second. Of course, if you were to utilize stress/intonation on any particular word in either case you could reverse the meaning easily, but without that element this is a good way to translate it.