Here is the first ‘history’ post to give you some idea of the Tower’s origins.
After William, Duke of Normandy, invaded England and defeated Harold at the Battle of Hastings in 1066, thereby becoming William the Conqueror, he knew that to rule England he had to rule London. After an initial stronghold was built on the south-east corner of the Roman city walls, work began on a great stone tower, the White Tower, which would dominate the landscape and proclaim William’s power as monarch.
Stone for the tower was brought from as far as France, while most of the physical building work was undertaken by English labour. When this stone tower was completed by 1100 its impact was as much pyschological as well as physical. Certainly, it was like nothing England had ever seen before. The Tower was never meant as a royal residence, though it would serve as one on numerous occasions, nor was it meant as the first line of defence following an invasion, but was intended as a fortress-stronghold, a role it would reprise repeatedly until the 19th century when its growth as a visitor attraction began to accelerate.
You can read more about the Tower’s origins here.