Today the River Fleet has been reduced to a trickle in a culvert under New Bridge Street that emerges under Blackfriars Bridge, but before the development of London it was the biggest river in the area, after the Thames. 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.
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. Montfichet's Tower was further north on Ludgate Hill overlooking the strategic route west, through Ludgate and over the Fleet, that would become Fleet Street.