Translation:The priests went to the city with soldiers.
How do you determine whether it "istis" is translated with "they went" or "they were"? It seems like they're just up to the context here, which means they should both be correct, yet Duolingo counted my answer using the latter wrong.
In order for the verb to mean "to be" you need the predicative in the nominative case. Here there is a dative, which indicates location and therefore matches with the meaning "to go."