The summit of the White Tower covers a space of 100 feet on the eastern side, by 113 on the north and south. The four turrets, the most conspicuous points in any view of the Tower, rise 16 feet above this leaden field, and each is crowned with pepper-box-shaped roofs made of lead. The turret crowning the south-eastern angle contains a chamber traditionally known as the prison of Joan of Kent. In the early years of the eighteenth century it was used as an observatory by Flambard, the Astronomer-Royal, and a contemporary of Isaac Newton, some years before the great Observatory was built at Greenwich. Although cannon were mounted on the roof in Tudor days, the platform could not have supported very heavy artillery, as it was only built of shingle. As I have said elsewhere, no record has come down to us of the time when the turrets with their little pepper-castor tops were first placed there, but the Harleian MSS. prove that similar ones existed as far back as the reign of Henry V. There is much difference of opinion as to the original mode of entrance into the White Tower. Probably the principal entrance lay on the south and river side of the Keep, near its western angle, for on the second floor there is a large opening on the exterior of the masonry which has parallel sides, and was doubtless formerly used as a doorway. Near this opening, and on the eastern side of the Keep, is a small door opening into the base of the well staircase. Both Mr Clark and Mr Birch believe that these doors formerly communicated with a building which stood on the south of the White Tower, having its outer entrance at the east end. This building would probably date back to the days of the Normans.