The Temple Mount forms the northern portion of a very narrow spur of hill that slopes sharply from north to south. Rising above the Kidron Valley to the east and Tyropoeon Valley to the west, its peak reaches a height of 740 m (2,428 ft) above sea level In around 19 BCE, Herod the Great extended the Mount's natural plateau by enclosing the area with four massive retaining walls and filling the voids. This artificial expansion resulted in a large flat expanse which today forms the eastern section of the Old City of Jerusalem. The trapezium shaped platform measures 488m along the west, 470m along the east, 315m along the north and 280m along the south, giving a total area of approximately 150,000 m2 (35.5 acres). The northern wall of the Mount, together with the northern section of the western wall, are hidden behind residential buildings. The southern section of the western flank is revealed and contains what is known as the Western Wall. The retaining walls on these two sides descend many meters below ground level. A northern portion of the western wall may be seen from within the Western Wall Tunnel, which was excavated through buildings adjacent to the platform. On the southern and eastern sides the walls are visible almost to their full height. The platform itself is separated from the rest of the Old City by the Tyropoeon Valley, though this once deep valley is now largely hidden beneath later deposits, and is imperceptible in places. The platform can be reached via Bridge Street — a street in the Muslim Quarter at the level of the platform, actually sitting on a monumental bridge; the bridge is no longer externally visible due to the change in ground level, but it may be seen from beneath via the Western Wall Tunnel