Archaeological excavations in 1986–90 uncovered waste pits and the remains of ditches between Carter Lane and Ludgate Hill.

It formed the western boundary of the Roman city of London and the strategic importance of the junction of the Fleet and the Thames means that the area was probably fortified from early times.

[1] The Normans reinforced the area by building two castles inside the Roman walls that ran north–south, giving their name to the street of Old Bailey and then roughly following the modern Blackfriars Lane down to the Thames.

Baynard's Castle was built where the wall met the river overlooking the mouth of the Fleet, roughly where the Bank of New York's Mellon Centre stands at 160 Queen Victoria Street.

[4] The 16th-century historian John Stow ascribes construction to a Baron of Mountfichet, who came to England during the Norman Conquest[5] – Montfiquet is a village in Normandy between Bayeux and Saint-Lô.

[3] Robert Fitzwalter of Baynard's Castle was the leader of the barons' revolt against King John which culminated in Magna Carta in 1215.

The tower was certainly in ruins by 1278, according to a deed drawn up between the Bishop of London, the Dean and Chapter of St Paul's and the Dominicans about the proposed locations of the new friary church.

A 1980s study of the topography of medieval London suggested a location bounded by Ludgate Hill and Carter Lane to the north and south, and St Paul's deanery and the city wall to the east and west respectively.