"It is thought to have originally been much larger, and (traditionally) to have been a Roman milestone, or possibly a Druidic altar. It is also said to be the milliarium, marking the central spot from which all distances were measured in Roman Britain. (The stone is very close to the centre of the east-west diameter of the City of London, as defined by the Roman walls).
The Stone sometimes called the Stone of Brutus, referring to legendary Trojan founder of London in around 1,000 BC. Popular legends include the stone being the remains of an ancient stone circle that is alleged to have stood on Ludgate Hill and even the stone from which King Arthur withdrew the legendary "Sword in the Stone" " Wikipedia