According to tradition, these steps (now covered with wooden plants) are the very ones Jesus climbed in the house of Pontius Pilate only to see the Roman governor wash his hands of the self-styled messiah. Emperor Constantine's mother St Helena brought these back in the fourth century. A crawl up the Scala Santa has been a fixture on every serious pilgrim's list ever since. In 1510 Martin Luther gave it a go, but halfway up he decided that relics were a theological irrelevance and walked back down again. Don't climb them unless you know 28 different prayers (one for each step); walking up is not allowed. Prepare for a queue on Good Friday.
At the top of the Holy Stairs (but also accessible by non-holy stairs to the left) is the Sancta Sanctorum ('Holy of Holies'), the privatissima chapel of the popes and one of the only monuments around here that escaped Sixtus V's revamping. Some of the best early Christian relics were kept in the crypt under the altar at one time - including the heads of saints Peter, Paul and young Agnes. Most of them have now been distributed to other churches around the city, but displayed in a glass case on the left wall is a fragment of the table on which the Last Supper was supposedly served. The real treasures here, however, are the exquisite 13th-century frescoes in the lunettes and on the ceiling, attributed to Cimabue. Once, no one but the pontifex maximus himself was allowed to set foot in the Sancta Sanctorum, but the exclusive entry policy has since been relaxed: anyone with €3.50 is welcome.