I also wrote it and got the same result. I'm curious because I remember seeing 'at' as a tooltip in other translations.
«The castle is in the market» is not accepted despite the hint on the word «rynku»
OK, we're gonna add it, but market is usually a place where you buy stuff, so it's rather hard to imagine a castle there... town square is definitely a better translation. "Rynek" is "market" only in some contexts, including the economy one.
Well, "rynek" is not technically a place for selling stuff, it can just take place there... but "market" was accepted, I guess okay, added.
There isn't any town hall in the sentence. It really is about a castle. Like a medieval one.
'Marketplace' for 'rynek' gets rejected even though it has been used in duolingo's own statements.