The home of emperors going back to the Ming dynasty, the Forbidden City in the centre of Beijing is listed by UNESCO as the largest collection of preserved ancient wooden structures in the world.

This one-off documentary goes into areas tourists never get to see, down the locked halls and into the mighty roofs, to reveal not just why the city was built but also how.

A sequence showing how an entire marble staircase might have been moved using an ice road is particularly impressive, and an experiment using a scale model of the 980-building complex shows how clever engineering protects the City from earthquakes.