Imagine a perfect square, 3 kilometers (1.8 miles) on each side, enclosed by a wall and, outside that, a wide moat. That describes the city of Angkor Thom, which was the Khmer capital for more than 400 years, starting in the late 1100s. Although the wall and moat remain, most of the city's buildings (made of wood) are gone. But at the dead center of that great square stands one of the most picturesque and evocative ruins of the Khmer empire: the former state temple of Bayon. Itself square, Bayon is a symbolic mountain composed of towers of various heights that converge on a central peak, built over a series of concentric square galleries. The galleries contain 14-foot-high walls carved from top to bottom with scenes from the Khmer kingdom, while the towers bear huge carved faces with enigmatic smiles, thought to represent the bodhisattva (enlightened being) of compassion. The combined effect is unforgettable, even amid the crowds of visitors.