I don't mean to step on the toes of most of the posters on this thread but I think a lot of the beauty and depth of Rome is lost on moderns because we don't understand the aim of Roman civilization. Rome since its beginning focused on channeling greatness, excellence. Ancient Romans believed that their city and nation was founded by the Gods to be an instrument of Justice and Nobility on earth. This drove the almost ridiculous need for Roman conquest of its neighbours.
Now, one of the most hallmark celebrations of Roman excellence was the magnificent display of their spoils of war or "trophies." The Romans would do this by bringing the "trophies" of victory back to the city of Rome and parading them around with great pomp. History records Roman generals parading through the city the leaders of conquered enemy armies and their personal weapons. This was also done with the great treasures of enemy civilisations. The obelisks, the rostra of enemy ships, the silver treasures from the Jєωιѕн temple, etc...
Now God did not annihilate Rome, He converted it. So what could be more fitting in the symbolism of our great religion than for all of the glory of the pagan city of Rome to now be subject to the true conqueror of the world, Christ? All of the trophies, skeletons, and spoils, indeed the skeleton of pagan Rome itself now lie at the feet of Roman Catholic Church. Even regarding the pagan religious monuments like the obelisks, which I am totally willing to believe are symbols of phallicism, I think are fittingly put on display in Rome, being topped by crosses. The pagans worshipped the phallus as the source of life, but this has been conquered by the Cross of Our Lord, the Tree of the Cross, true source of life.