As you might know, entry to the Square Mile was once controlled through seven gates: Aldgate, Bishopsgate, Moorgate, Cripplegate, Aldersgate, Newgate and Ludgate. All were demolished in the 18th century, leaving little trace.
But a small fragment of Ludgate may still survive, just yards from its original location.
Turn south off Ludgate Hill, down the short side-road called Pilgrim Street. Immediately on your right, you'll find this rounded lump of stone by the kerb.
It's clearly not a structural feature of the building, and serves little purpose as a bollard. Could it be a remnant of the old Ludgate, which once stood directly next to this spot?
A 1953 guide to the capital makes the suggestion. The wonderful London Explorer by Peter Jackson provides a drawing of the stone and Ludgate, with the note: "This is said to have been one of its cornerstones."
Not conclusive, then, but plenty of circumstantial evidence.
Another, well known fragment of the gate can be found nearby on Fleet Street. The church of St Dunstan-in-the-West includes statues to King Lud and his sons, plus Queen Elizabeth I. All once stood on the gateway.