"There is a castle in my town."
Translation:W moim mieście jest zamek.
I think that would be more like "It is a castle in my town". or "this is/ that is a castle in my town"
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.
I need an explanation on why it can't be w moim mieście ma zamek. I thought mieć could also be used to mean "there is".
It is used as "there isn't"= "nie ma".
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.