The north-east corner tower

In front of you, on the corner of the wall and reflected in the moat, is one of the four corner towers of the Forbidden City. They are identical, and all four were built with the palace, around 1420. They were not just for keeping watch: they were also a showpiece of carpentry.

1A puzzle of roofs

Look at the roof. Three levels of eaves cross, fold and overlap, as if several houses had been slotted together. The whole structure is jointed timber set on top of the wall. Beijingers sum it up with a saying: “nine beams, eighteen pillars, seventy-two ridges”.

2The carpenter and the cage

A legend grew out of that saying. It is said that the Yongle Emperor demanded towers of exactly that form from his carpenters, on pain of death. Nobody knew how to do it, until an old man came by selling cricket cages made of bamboo sticks. One of them had exactly nine beams, eighteen pillars and seventy-two ridges. The master builder took it as his model and saved his life.

According to tradition, the old man was Lu Ban, the legendary carpenter and patron of Chinese builders.

The numbers in the saying are more a way of expressing complexity than an exact count of parts. The moat, about 52 metres wide, surrounds a wall some 3.4 kilometres long.

3What to look for

• The crossing eaves: try counting the ridges.
• The gilded finial at the very top of the roof.
• The reflection in the moat, especially at sunset.