What is the "city of seven hills" in St. Malachy's Popes Prophecy?

Reading through St. Malachy's Popes Prophecy , I couldn't understand a particular phrase: > Peter the Roman, who will nourish the sheep in many tribulations; when > they are finished, **the city of seven hills will be destroyed**, and the > dreadful judge will judge his people I have been searching the internet for some time now but I have not found any clue. What does _the city of seven hills_ refer to?