Constantine gave the old cemetery to the Church and arranged for the hill to be flattened to build the basilica which was built with its altar directly above St Peter’s tomb (as it still is in the new basilica built in the 16th century). The circus was not donated to the Church and remained in use (although the public executions and later the pagan blood sports were later banned and it was used only for non-violent activities. )

The new basilica is so large that one of its side-arms (to the left of the altar as you face it from the entrance) covers much of the site of the old circus, and the Egyptian obelisk which had stood on its spine was painstakingly transferred to St Peter’s Square where it now stands.

Fascinatingly, this obelisk was not only standing beside St Peter when he died, it had also been standing in Lower Egypt when Moses led the Israelites past it out of slavery.

And St Peter’s is not “the seat of the papacy”. The pope’s official cathedral church is the Lateran basilica of St John the Apostle/Evangelist and St John the Baptist, on the other side of ancient Rome, built on land which had been an army barracks which was also donated to the church by Constantine.

St Peter’s is merely a very very large ordinary church. (Though technically no longer the largest church in the world, there are now larger ones in Lourdes and in Ghana.)