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".
That's the full name, actually. "The Great Wall" is the commonly used and generally more idiomatic short form for the reasons you mentioned.
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.
Is it still wrong without the "very" for large? Duolingo usually adds 很 to make it sound more grammatically correct.