Have you ever wondered what's so forbidden about the Forbidden City? It's called that because it was closed to the outside world for 500 years. This was the seat of the Ming and the Qing emperors, and no one could enter - or leave - the imperial domain without their permission. These days, the Chinese mainly call it Gu Gong, or Former Palace.
The Forbidden City, or Beijing Imperial Palace, is BIG - you'll need to allow at least one day for your visit. UNESCO have listed it as the largest collection of ancient wooden structures in the world. There are nearly 1,000 rooms in over 800 buildings. However, because it's been ransacked by invaders and gutted by fire several times (wooden buildings, lanterns, you do the math) most of the structures date from the 18th century on.