Altdorf is the capital city of The Empire, and the entire Order, from where Emperor Karl Franz commands the Imperial army from his palace overlooking the city. It is here in Reikland's heart that the Order of the Griffon was founded and here that it will be destroyed, if Tchar'zanek succeeds in his war against the Empire.
Tome of Knowledge
Altdorf and The Inevitable City were the third and fourth cities to be be built. Altdorf ended up being vastly different from its initial design. This is possibly after Mythic decided to focus on only 2 Capital Cities but make them bigger and better.
You can find a player created list of Quest Guides for Altdorf by Clicking here.