There are no answer choices, but I already know why Constantinople is a wonderful fort. All of the following are true regarding the city's location:
It's on the Bosphorus strait, allowing for easy defense on its Eastern border and regulation of trade (spices had to come through this city to reach Europe).
It was a peninsula on top of a rocky plateau, meaning that three sides of the city were surrounded by water (can't invade a mountain on a boat) and the remaining side on land was very heavily fortified.