How do I get inside the Forbidden City?

Hint 1 of 4:

You can't get in unless the Emperor thinks he owes you a favor.

Hint 2 of 4:

A favor of one of his ancestors might work just as well.

Hint 3 of 4:

Have you gotten the Emporer's seal from 44 BC?

Hint 4 of 4:

You'll need to free the Great Wall from the invaders.
(See the question in Peking, 44 BC, for more help.)