A source of perpetual confusion to foreigners is the fact that the City of London is not the whole of London, but a tiny enclave inside it.

Known as “the Square Mile”, which is roughly its area (2.9km²), the City isn’t even in principle part of Greater London, and officially forms its own separate county. And it isn’t a London borough, but an entity in its own right, independent of the city (with a small “c”) that surrounds it.

All this special status stuff dates back to Norman times. The City was originally the walled Roman city of Londinium. The Anglo-Saxons called it Lundenburgh, to distinguish it from Lundenwic, their new settlement around Covent Garden. With the coming of the Vikings, the burh came into its own again, offering walled protection which the wic did not, and when William the Conqueror defeated the Anglo-Saxons at the Battle of Hastings in 1066 and marched on London, his only access to the walled city from the south was by fighting his way across London Bridge. Balking at that, he went round London and approached it from the north. The surrounded and outnumbered Londoners cut a deal: recognize our established rights as an autonomous city, they said, and we’ll accept you as king. William agreed, and that was the basis of the City’s special status. Legally, it’s almost as much a separate UK entity as England, Scotland or Wales.

Still suspicious of London, the Normans built three castles to control it. While the Roman citadel had been the Barbican, up by Cripplegate, the Normans’ new citadel was the Tower, whose oldest part, the White Tower, is a typical square Norman castle. Their other two castles were Baynard’s and Montfichet's on the City’s west side, now long gone. But despite their suspicions, the Normans kept their end of the deal, and in 1215, when King John signed the Magna Carta, one of its clauses provided that “The City of London shall have all its ancient liberties by land as well as by water.”

The City was built around the Walbrook, a stream which used to run into the Thames. The street called Walbrook runs along part of its ancient course. The Walbrook divided the City into two halves, both hills: Ludgate Hill to its west, topped by St Paul’s Cathedral; and Cornhill to its east, topped by the church of St Peter. The City ended on its western side at Ludgate, halfway up Ludgate Hill, but in 1282, the southern end was extended down to the River Fleet (now New Bridge Street), so as to take in the Dominican priory at Blackfriars.

Even by then, the City’s jurisdiction extended beyond its walls, and toll bars set up outside the walls, to control the flow of goods in and out of the City, became its new boundaries in place of the wall’s seven gates. As a result, the City grew, especially to the north and west, with many of its wards expanding to include sections “within” and “without” the walls, which often then separated. So Farringdon, for example, split into the wards of Farringdon Within and Farringdon Without. This expansion took in parts of areas such as Holborn and Finsbury, which is why parts of those neighbourhoods are inside the City (“below the bars”) rather than “above the bars” in their own respective boroughs. Nowadays, dragon statues have replaced the former bars.

Temple Bar, where the Strand meets Fleet Street, was the most famous of the City’s six boundary bars, but there were others at Holborn, Smithfield, Aldersgate, Spitalfields (Bishopsgate) and Whitechapel (Aldgate). Christopher Wren’s seventeenth century Temple Bar arch was exiled in 1878 to a park in Hertfordshire, and only came back to the City in 2001, not in its original position, but off St Paul’s Churchyard. The City’s boundaries were tinkered with again in 1994.

Although it’s the oldest part of town, the City today seems almost entirely modern. That’s because of two events which largely destroyed it: the Great Fire of 1666, and the Blitz of 1940–41. St Paul’s burnt down in the Fire, but Wren’s replacement famously – and narrowly – escaped destruction in the Blitz. Very few things survived both calamaties, but they include the fifteenth-century Guildhall, the thirteenth-century Church of St Etheldreda in Holborn, and the sixteenth-century Hoop and Grapes pub at Aldgate.

The City is run by a corporation, which is chosen mostly by City-based businesses (their bosses, not the employees who actually work in the Square Mile), and by ancient “livery companies” (medieval guilds), with meagre representation of the City’s 9,000-odd residents. It has a special lobbyist called the Remembrancer to represent it inside the UK’s House of Commons. The City uses its privileges and autonomy to sit at the centre of a web of off-shore tax havens providing refuge for the gains of tax avoiders, corrupt politicians and organized crime worldwide, at the expense of pretty much everybody else in the world. Meanwhile, having become London’s financial district, the City was Europe’s banking and finance centre before Brexit, and the world’s financial capital until Brexit shifted that to New York.

At the same time, the City has shut the actual metropolis of London out of its ancient privileges, which are confined purely to the Square Mile. In the seventeenth century, London’s population was swelled by people displaced as landowners enclosed and effectively seized more and more of England’s common land. These refugees moved into areas around the City, but the rich people and institutions within its limits resisted moves to extend their jurisdiction (and duty of care) to the poor in these new suburbs, and in the “Great Refusal” of 1637, the City definitively said no to extending its borders to cover the whole of London, establishing it as the corrupt, undemocratic, super-rich little enclave that it is today.