At the four corners of the Forbidden City are four watch towers. Each of the four watch towers has 9 roof beams, 18 pillars and 72 ridgepoles. You can rarely see such structures elsewhere. The watch towers are ingeniously built using the magic number "9". Each of the three numbers are either 9 or multiples of 9, And the total of the three numbers is 9+18+72=99, which is a heavenly number only the Forbidden City is worthy of.

But who designed the watch towers and how?

Legend has it that when the city of Beijing was built, there were no watch towers, only the high, thick red wall. One night, the emperor dreamed of four watch towers standing at each corner. When he woke up he ordered that the watch towers should be made like the ones in his dream.

The Minister of Works accepted the imperial edict and summoned the best craftsmen in the country to build the watch towers in three months.

Two shifts of the best craftsmen built very impressive watch towers, but because they weren't like what the emperor had dreamed of, they were all destroyed and the craftsmen were all beheaded

The third group of craftsmen was summoned. They were so worried that they could hardly eat or sleep.

One day while they were lying down chatting, they heard the singing of grasshoppers. It was so noisy that one craftsman got fed up after a while and went out and prepared to take some action. He saw that an old man was selling those grasshoppers for a very high price. The craftsman hoped to get some quiet, but the old man said, how can a grasshopper keep silent? The craftsman argued with the old man, and all the craftsman went out to see what was happening. Upon seeing these craftsmen, the old man held his grasshopper cage high and said loudly, "Sir, won't you buy my fine grasshopper in his fine cage?" All eyes focused on the cage, it was exactly what they wanted!

The cage was made of thin sorghum stalks with eight sides and eight more at every corner. The roof was divided into three stories. The first story had upturned eaves on its four sides, and each of the four corners also had four small upturned eaves. The second was similar to the first one, except that on its four small upturned eaves were four triangular ridgepoles. Like the second, the third had four triangular ridgepoles, but on the ridgepoles were four more triangular ridgepoles, and on each of them stood four level ridgepoles.

The craftsmen built the four watch towers according to the design of the cage, and the emperor was finally quite satisfied.

The old man was believed to be Lu Ban Grandfather of all Chinese carpenters.