The Duo grammar notes say that Latin has no fixed word order and that the "ne" particle should be added to end of the first word of the sentence. So my question is: would it be possible to say 'Civitasne est?"
Because the word order can be switched around, your response is correct. As long as the -ne is attached to the first word and not any other. Don't forget that the -ne is not required. So "Civitas est?" is also correct.