"Our city has an old castle."
Translation:La nostra città ha un vecchio castello.
I wrote "La nostra città ha un castello vecchio" which was correct too, what is the difference for the placing of the adjective vecchio here?
In this case it is just matter of emphasis. If the adjective is placed before the noun, we are giving greater emphasis to the noun, if it is after the noun, we are giving greater emphasis to the adjective.
In some other cases the position of the adjective is more important.
I'd translate old castle to Portuguese as if it was "antico", it's much more natural.
Why is the "La" at the beginning necessary? Or is it just that the alternative translations are not in here yet?
The appropriate article is always used with a possessive, so "our city" is "la nostra città". (Singe family members are the one exception I know of - it's "mia madre", not "la mia madre".)