Translation:The Great Wall of China is long, the Forbidden City is big.
When you hear "the Great Wall" you automatically think "of China". Wouldn't referring back to the country be redundant because I don't know of any other country having "The Great Wall"? I'm American and my friends all refer to "The Great Wall of China" as "The Great Wall".
Actually it's a mistake that even Chinese do a lot. The Forbidden City and 故宫 are not exactly the same thing. The Forbidden City is 紫禁城 (Purple Forbidden City where purple refers to the North Star). Whereas 故宫 is the Imperial Palace that is within the walls. There are two Imperial Palaces in China: one in Beijing (北京故宫) and one in Shenyang (沈阳故宫). Surprisingly the Chinese Wikipedia makes this difference but not the English Wikipedia.
故官 or 禁城：
故 - ancient
官 - palace, court
禁 - forbidden
城 - city
It used to be the residence of kings and there could be no trees so there were no places for assassins to hide (among other reasons). It is "ancient" palace complex as it was completed in 1420 and has been the former Chinese imperial palace from the Ming dynasty to the end of the Qing dynasty, and today houses the Palace Museum.