The Romans were not the first to set foot on the shores of the Thames. The great Celtic King Lludd Llaw Eraint fortified one of the settlements on the river, building a tower and defending the people from neighboring tribes.  Legends say he captured dragons and tricked magicians, and perhaps this is true, for he ruled over a time of peace and property.  When he died, the city was named London, after him, and a temple was built in his honor.  Centuries later, the Ludgate would be named for him, and is rumored to be built over his final resting place.

When the Romans arrived, they built Londinium on the old Welsh city. This first settlement was destroyed by the Iceni, but fourty years later, in 100 AD, a new city was built as as the capital of the Roman province of Britannia. At its height in the 2nd century, Roman London had a population of around 60,000. To defend the city, the Romans built the London Wall, a large fortification that enclosed over three hundred acres. The wall initally had six gates, but years later the Saxons added a seventh.

The city suffered greatly under the collapse of the Roman Empire, having been virtually abandoned by its inhabitants.  Still, its location on the Thames was too good for it not to eventually be reclaimed. By 640, it began to expand westward beyond its walls, and a few generations later was a central market for many people coming by land or by sea. Soon, this lead to attention from the Saxon kings – they built a monastery at Westminster and a royal palace in the ruins of an old Roman amphitheater.  The royal palace would eventually come to be known as the Guildhall, an administrative center of the city.

The wealth that London amassed as a trading center drew the attraction of the Vikings, who sailed up the Thames and attacked the city. Starting around 830, the city suffered from multiple Danish assaults. And then, in 865, the “Great Heathen Army” of Vikings invaded and spent the winter of 871 to 872 within the walls of London. By 878 King Alfred the Great had become the King of all the English, and forced the Viking leaders to sue for peace. Eight years later he re-established London within the old Roman city walls, as one of a system of defensive burghs around the country. By 920, the London had been rebuilt completely and became the most important commercial center in England, attracting traders from across the known world.

The Viking attacks upon the city ceased when Canute came to power in 1017. When Canute died, the city passed to the Saxon Edward the Confessor, who is perhaps best known for re-founding the great Abbey at Westminster, along with the adjoining palace, and moving his court there. London had become the largest and most prosperous city on the isle of Britain, even if it was not the official capital of the realm.

Upon Edward's death, Harold was crowned king in Westminster Abbey, setting a tradition that would continue for generations to come. Harold was defeated by William the Conqueror at the Battle of Hastings. The officials of London quickly recognized William as King, and thus he granted the city the first formal charter of his reign. Three months after his victory at the Battle of Hastings, William was also crowed at Westminster.

William built a castle at the water gate on the western edge of the city wall that would eventually become known as simply the Tower of London. It acted as the royal residence, royal mint, and treasury. The city thus became the seat of power for the English kings, and its walls were bursting with an ever growing population of both Saxons and Normans.  William also oversaw construction of two other castles: Montfichet's Castle and Baynard’s Castle. 

In 1087 construction began on St. Paul's Cathedral after a devastating fire destroyed much of the city.  Nearly a hundred years later, work continues on the Cathedral, but already it is considered one of the largest and most striking church in England, if not Europe.  The nave's immense length was particularly notable, with a Norman triforium and vaulted ceiling. The length earned it the nickname "Paul's walk".  The site of the Cathedral is said to upon the same hill on which once stood the temple to Lludd.

Priory Church of St Bartholomew-the-Great was established in 1123 by Rahere, a prebendary of St Paul's Cathedral and later an Augustinian canon, who is said to have erected the church in gratitude after recovering from a fever. Rahere's supposedly miraculous recovery contributed to the church becoming known for its curative powers, with sick people filling its aisles each St Bartholomew's Day.  Eventually, a hospital was built next to the priory to care for the sick. 

More recently completed is the Temple Church, located on the Thames just outside of the Wall, which was built by the Knights Templar as their English headquarters. In addition to the church, the Temple compound containes residences, military training facilities, and recreational grounds for the military brethren and novices, who were not permitted to go into the city without the permission of the Master of the Temple.

In 1176 the first stone London Bridge was built, mere yards from the original Roman bridge across the Thames. Because the passage across the bridge was narrow and clogged with traffic, it was much quicker and easier for travelers to hire waterboatmen to row them across the river. The city itself was a confusing maze of twisting streets and lanes.  By the time Henry II was crowed in Westminster as Rex Angliae, the city had nearly 20,000 inhabitants.