The Forbidden City is surrounded by a 7.9 metres (26 ft) high city wall and a 6 metres (20 ft) deep by 52 metres (171 ft) wide moat. The walls are 8.62 metres (28.3 ft) wide at the base, tapering to 6.66 metres (21.9 ft) at the top. These walls served as both defensive walls and retaining walls for the palace. They were constructed with a rammed earth core, and surfaced with three layers of specially baked bricks on both sides, with the interstices filled with mortar.