When Was the Tower of London Built? William the Conqueror's Fortress on the Thames

The Tower started as an occupation. William the Conqueror landed in 1066, took London, and by the end of that year had begun a fortress in the corner of the old city walls, where it could watch both the river and the people living beside it. The first defences were earth and timber, quick to raise and meant to last only until something stronger could replace them.

The something stronger was the great stone keep. Work on the White Tower is usually dated to 1078, under Gundulf, the Bishop of Rochester, whom William put in charge of the building. It measures 36 by 32 metres at the base and rises 27 metres, big enough to hold royal rooms and a chapel, St John's, which is still there inside the keep.

It was built to be seen as much as to be defended. For Londoners used to timber and thatch, a stone tower on this scale was a statement about who now ran the country, and it stayed the heart of the fortress through every later century of building around it.

The short answer is 1078 for the White Tower, but the Tower of London as it looks today took well over two hundred years. Richard I, in the 1190s, dug the first moat and doubled the size of the castle, pushing its defences out well beyond the old keep and its first ring of walls.

Henry III, who reigned from 1216 to 1272, turned the inner ward into a palace, with new royal lodgings beside the military buildings. His son Edward I finished the job: between 1275 and 1285 he spent about £21,000 on an outer ward and a second ring of walls, turning the Tower into a concentric castle, one line of defence inside another.

Edward's works also gave the Tower its river entrance, a water gate under St Thomas's Tower that later became known as Traitors' Gate. The fortress took on new jobs at the same time: the royal mint was brought together here in 1279, and the Tower of London zoo, a royal menagerie of lions and other animals grew up inside the walls.

From the Middle Ages onward the Tower was also England's most famous prison. Prisoners brought by river came in through the water gate, and it has been called Traitors' Gate since before 1543. Sir Thomas More was one of the men who passed through it on the way to his cell.

Its darkest mystery is the Princes in the Tower. Edward V and his brother Richard were kept here in 1483, aged 12 and 9, and never seen again after that summer, while their uncle took the throne as Richard III. In 1674 workmen found two small skeletons in a wooden box beneath a staircase, and Charles II had them buried in Westminster Abbey. Whether the boys were murdered, and by whom, is still argued over.

Executions inside the walls were rarer than the legends suggest. Only seven people were executed within the Tower before the twentieth century, among them Anne Boleyn, beheaded on 19 May 1536 by a swordsman brought over from France. The last person executed at the Tower was the German spy Josef Jakobs, shot on 15 August 1941.