The first bridge over the Thames was buikt by the Romans about AD 50, or some 2000 years ago, says Wikipedia.
The King John version of the bridge completed in 1209. He licensed the building of houses on the bridge, as a direct means of deriving revenue for its maintenance, and it was soon colonised by shops.
The medieval bridge had 19 small arches and a drawbridge with a defensive gatehouse at the southern end. Contemporary pictures show it crowded with buildings of up to seven stories in height.
The buildings slowed down the traffic crossing the river. The houses and shops took up space and could draw crowds, and when carts broke down or animals misbehaved, crossing the bridge could take up to an hour. For this reason, people on foot often chose to use the dozens of river taxi boats that quickly ferried Londoners from shore to shore.
Nearly 200 places of business lined both sides of the narrow street. Ale and beer were not sold on the London bridge because these beverages required cellars, which were not present. The merchants lived above their shops and sold goods from the street-level floor. They used windows to show their goods and transact business; over each shop hung a sign usually in the shape of the articles sold, in order that the illiterate could recognise the nature of the business. These signs were posted high enough that a rider on a horse could pass beneath them— every inch of the small street had to be available to vehicular traffic. Many of the top floors of the houses and shops were built over the street and actually connected to the house or shop across the street, giving the street a tunnel look.
The southern gatehouse, the Stone Gateway, became the scene of one of London's most notorious sights: a display of the severed heads of traitors, impaled on pikes[1] and dipped in tar to preserve them against the elements. The head of William Wallace was the first to appear on the gate, in 1305, starting a tradition that was to continue for another 355 years. Other famous heads on pikes included those of Jack Cade in 1450, Thomas More in 1535, Bishop John Fisher in the same year, and Thomas Cromwell in 1540. In 1598 a German visitor to London Paul Hentzner counted over 30 heads on the bridge.
The buildings on London Bridge created a major fire hazard and served to increase the load on its arches, both of which may have contributed to the several disasters on the bridge. In 1212, perhaps the greatest of the early fires of London broke out on both ends of the bridge simultaneously, trapping many in the middle and reportedly resulting in the death of 3,000 people. Another major fire broke out in 1633, destroying the northern third of the bridge, although this prevented the bridge from being damaged by the Great Fire of London in 1666.
In 1756 permission was obtained to demolish all the shops and houses on London Bridge.
By the end of the 18th century, it was apparent that the old London Bridge — by then over 600 years old — needed to be replaced. It was narrow and decrepit, and blocked river traffic. The bridge was eventually replaced by a structure of five stone arches.