The castle is situated on a chalk mound overlooking the River Thames.
It was founded by William the Conqueror in around 1075 to protect the western approaches to London.
Unlike other Norman castles it may have built in stone from the start as the natural mound was able to withstand the great weight of a stone keep. Local sandstone was used and at the same time two enormous baileys were laid out to the east and west.
The castle is the oldest continuously occupied royal residence in Britain and a great many monarchs between the 12th and 20th century have made modifications and additions to the site.
In 1175 Henry II rebuilt the shell keep and added curtain walls fortified with towers around the baileys. The house he had built himself forms the basement of the present day royal apartments. King John caused the only two sieges in the castle's history and Henry III inherited the task of carrying out the considerable restoration.
Edward III added domestic buildings and instigated the Order of the Garter.The Knights of the Garter claim St George's Chapel as their traditional place of worship. The chapel was begun by Edward IV in 1477 but the building work was so spectacular that it wasn't completed until 1528 during the reign of Henry VII. Henry VIII built the gatehouse named after him in 1511.
Charles II constructed the three-mile avenue known as the Long Walk and several buildings in the Upper Ward.
However, there was no other serious building work until George IV had the castle modernized in the 1820's. He employed Sir Jeffrey Wyatville to improve the domestic buildings and provide more room for servants by raising the number of storeys of the Upper Ward. The Round Tower was also raised in height and the flag turret added.
No other major work was carried out until 1993 when a devastating fire broke out during maintenance work on the State Apartments. St George's Hall was destroyed by the fire but has since been rebuilt.