Look for: Missing marble. The Colosseum was once clad in travertine marble, giving it a white sheen. After the fall of the Roman Empire, the marble was used for other buildings. The front steps of St. Peter's Basilica include marble from the Colosseum.
The Christians liked to appropriate what Emperor Hadrian built. This was opened in 139 as a mausoleum for Hadrian and his family, but by medieval times Hadrian had been evicted and his cylindrical tomb towering over the Tiber River turned into a convenient fortress for pope-fleeing invaders descending on Rome or urban riots welling up within the city walls. An elevated walkway was built from the papal apartment in St. Peter's to Castel Sant'Angelo, allowing pontiffs to flee to a stronghold without having to set foot on city streets. Today, it is a museum with a popular rooftop café that's the perfect setting to gaze out at Rome on a warm day.
Look for: Pons Aelius. The second-century bridge that stretches from Castel Sant'Angelo across the Tiber to the old city of Rome. Known today at Ponte Sant'Angelo, it's a much simpler space than in the distant past.
Homes and a triumphal arch once stood on the bridge, until its foundations began to crack and the extra structures were stripped away in the 17th century. Today, it is a pedestrian-only space popular with artists and newlyweds.
For more than 1,500 years, the papacy wasn't just a spiritual power but a temporal one. It was the church of a great empire. By the late Middle Ages, the popes were rulers of the Papal States, which covered thousands of miles of land in and around Rome. With an estimated 800 churches in the city, as well as dozens of convents, schools, fountains, towers and other sites, there is little that was not touched by the papacy. It all changed in 1870 when the unification of Italy secularized all of the papal holdings except for the area immediately around St. Peter's. It would take another half-century and the intervention of a dictator to end the squabble between church and state.
ARCHBASILICAOF ST. JOHN LATERAN
Well outside the tourist areas of Rome is the most overlooked historic attraction in the Eternal City. It is the seat of the bishop of Rome, aka the pope, making it the official home church of the leader of Catholicism.
Built on the site of a palace from the time of Emperor Nero, the complex that included a palace and chapels was built over the decades to house the popes, most of whom lived here until the 14th century. A basilica has stood on the site since the fourth century, though the current baroque-style facade dates to 1735. The oldest element of the complex is the largest obelisk in the world - an Egyptian treasure dating to the 15th century B.C. that was taken to Rome and stood in the Circus Maximus until 1870.
The church was where popes were crowned. But with the occupation of Rome by Italian forces uniting the kingdom, popes refused to use the church - Pius XI refused to even leave the Vatican, claiming that he was a prisoner. An uneasy truce was maintained for 59 years until dictator Benito Mussolini hammered out the Lateran Treaty in 1929 between the Kingdom of Italy and the Holy See. It created the Vatican State and exempted church holdings outside of the Vatican from taxes. Though much of the statesmanship of the Fascist era was later repealed, the Lateran Treaty remains in force and is the reason the Vatican has a nonvoting seat in the United Nations.
Look for: The bronze doors. These massive, second-century doors were taken from the ancient Roman Senate building, an example of the frequent "repurposing" of classic architectural and design elements onto new buildings.
THE SACRED STEPS
Among the greatest holy relics of the church, the Sacred Steps are believed by the faithful to be the marble stairs that Jesus climbed to see Pontius Pilate on his way to crucifixion. Helena, the mother of the first Christian emperor, Constantine, visited the Holy Land from 326 to 328 and returned with several relics, including the 28 steps.
Today, the steps are housed in a chapel used by the pope just across the street from the Lateran cathedral. They are covered in wood, except for small holes left to show spots believed to be the blood of Christ. The faithful climb the steps on their knees. At the top is the Sanctum Sanctorum, the personal chapel of the popes when they resided at the nearby Lateran Palace.
Look for: A warning. A sign at the bottom reminds penitents that it is not enough to simply make the journey to the top - confession to a priest is required for the full benefits of the arduous climb to be fulfilled.
PIAZZA DEL POPOLO
Rome is roughly halfway down the peninsula of Italy. For most pilgrims and dignitaries, the city was entered from the north gate, via this imposing early 19th-century plaza, the last of a series of triumphant entryways to the city. The name has nothing to do with popes, but rather the poplar trees. Yet it was as important as any symbol of the power of the papacy.
On one side of the north gate were the villages and farms of rural Italy. On the other, a massive piazza with a 3,300-year-old Egyptian obelisk at its center, flanked by two large churches. Radiating out from the plaza were three roads, which led to the Vatican, the city center and the riverside of the Tiber. It was a place meant to startle those arriving with the power and majesty of the popes. Until 1826, it was the spot for public executions of criminals. Today, most visitors arrive in Rome via Leonardo da Vinci Airport to the south, essentially entering through the historical back door of Rome.