In German, there are some prepositions that can take either the dative (einem) or the accusative (einen) and each of these means something slightly different. As a general rule, use the accusative to mean movement toward or into, "in ein Schloss" = "into a castle", "auf den Tisch" = "onto the table" and dative to mean it's more literal English translation, "in einem Schloss" = "in a castle", "auf dem Tisch" = "on the table."
Hope this helps! For more info, google "dual-way prepositions."
"Schloß" is the old spelling, and "Schloss" is the new spelling.
According to the new spelling rules, "ß" is only used after long vowels and diphthongs (= "ei", "au", "eu" ... ). The "o" in "Schloss" is short, therefore you can't use "ß".
Is there a different way of saying "Did you go into a castle?", or can "Sind Sie in ein Schloss gegangen?" mean both that and "Have you gone into a castle?"?
Deepl and Google translate both sentences to the same German sentence.
Whereas those two sentences mean quite different things in English.
'Gingen sie in ein Schloss' would be possible, but for some reason, perfect tense is mostly used in German for both English perfect and English simple past tense. It would be interesting to have some German specialist elaborate on this, explaining the use of the two tenses. 'Waren Sie Mal zu einem Schloss" on the other hand sounds good to me. 'Sein' is often used in Imperfect/Pretäritum (simple past).