"In" with a noun in the accusative case, urbem, signifies motion. Just going "to" a city could be standing outside the city and looking at it. "In" carries the notion of "going in," especially with the accusative case.
The word in mean in or on if it takes the ablative case and it means into or onto if it takes the accusative case.
I answered "Corinna comes to the city" and it was marked incorrect. What is the difference between to and into here ?
Just as in English. "To the city" ("ad urbem") may mean that she simply walks up to the city walls and stops, whereas "into the city" means that she actually enters.