From the years 1000 to 1500, London was a typical medieval city with a population that expanded and contracted many times for many different reasons.
In 1067, William the Conqueror was crowned King of England. He started a building boom that included the Tower of London in 1097 and Windsor Castle in 1100. This boom was slowed by a large fire in 1200 which destroyed much of the city. The result of this fire was a law stating that the first story of all houses must be built of brick or stone.
The population of London continued to grow quickly in spite of the dirty living conditions and the lack of clean water in the city. This problem may have caused the Black Plague, which killed half the population of London in 1350.
During the Medieval period, London was a dirty, struggling, pre- industrial capital, but between 1500 and 1800 it developed into a modern, world city.
Information from London: A Concise History, by Geoffrey Trease