The Forbidden City
The internal architecture of the Fire Nation Royal Palace is drawn from the Palace of Heavenly Purity, the largest of the three halls of the Inner Court's of China's Forbidden City, the enormous imperial palace-city of the mid-Ming and the Qing Dynasties. The Inner Court was where the Emperor worked and resided with his family, eunuchs and maid-servants, much like that of the the Fire Lord and the royal family.
Avatar co-creators Mike DiMartino and Bryan Konietzko had the opportunity just last year to travel to Beijing, China and visit the Forbidden City as well as other parts of China. The two took several pictures to be later used as reference for the backgrounds seen on the show.