Thank you for the explanation. Very well, I shall use 'the castle' from now on on Duolingo. Yes, definitely, the differences between the various types of residences are irrelevant for the purposes of this course. While I do not understand how accepting 'the château' would be confusing, since the castle would still remain the correct and recommended answer, I understand that it is a very infrequently used word and therefore the probability of learners using it instead of the castle is quite low.
Duolingo's backend system unfortunately very frequently decides what to show as the correct translation entirely on its own, rather than based on what we have deemed the suitable defaults. As long as this is the case, we find that learners who do not speak native English are too often shown translations that are way above their fluency level, or only suitable in fringe scenarios - etc.
We'd very much like not having to take that into account, but, well - the reality is we do have to, I'm afraid.