Accorsding to https://en.pons.com/translate?q=Schlossl=deenin=lf=de,
Schloss can mean:
clasp (an einer Handtasche)
buckle (an einem Rucksack)
For examples of other ways to use the word Schloss, see https://en.pons.com/translate?q=Schlossl=deenin=lf=de#examples
"Ist das ein Schloss?"
Generally, 'das' is used for both 'this' and 'that'. There is actually a closer equivalent to 'this', 'dies', but it's not a word people actually use in speech.
If you specifically need the meaning of 'that', you can say 'das da' ('that over there', but without the emphasis on location).
I recommend for you this app https://play.google.com/store/apps/details?id=com.lubosmikusiak.articuli.derdiedas
TL;DR: "Schloss" doesn't translate as "castle"
Looking in the Longman Dictionary of Contemporary English:
Castle: a very large strong building, built in the past as a safe place that could be easily defended against attack// --Edinburgh Castle// --a ruined castle//
This doesn't really fit a meaning of "Schloss" in any way. See wikipedia:
Schloss (German pronunciation: [ˈʃlɔs]; pl. Schlösser), formerly written Schloß, is the German term for a building similar to a château, palace, or manor house; or what in the British Isles would be known as a stately home or country house.