It was the Russian course that taught me how different are Замок в моим городе and В моим городе (есть) замок. First one is about the castle, second is about the city. Same applies for Polish.
you can also use say "Mamy zamek w mieście" = we have a castle in town, ot Moje miasto ma zamek - Our town has a castle. But you need some subject in the sentence.