PBS has a great video, SECRETS OF THE FORBIDDEN CITY, in which "NOVA joins a team of master crasftesman (today) to build a scale model of a typical palace in a seismic lab, then subject it to simulated earthquakes to shake out the secrets of how the Forbiddden City has withstood centuies of Violent tremors."
Essentially, the entire structure has no nails, bolts, screws, cement etc.-- just brackets that work miracles!