The Citadel had a mixture of inspirations. The first, was a view of Preston from the Ribble Valley, perched on the hill above the river, looking quite like a modern citadel. That idea led to the concept of a citadel with a king or lord in it and a village below with ordinary people. I added to that something of a cross between the hammer horror style legend of Elizabeth Báthory, the Countess alleged to have bathed in virgin’s blood to retain her youth, and the ‘Child catcher’ of Chitty Chitty Bang Bang. I originally intended that the emperor would be a child himself, maybe a young teenager, but decided instead to have him look about Chrístõ’s own apparent age in appearance and in actual fact somec twenty years older. Another loose inspiration was the vampyres of Terry Pratchett’s Carpe Jugulum who ‘milked’ the people of the towns around their castle for blood while allowing them to live lives of servitude under them. Between those various ideas came this story.

Chrístõ’s ability to look into the mind of the Emperor and trace the kernel of good in him, is an element of his character that was commented upon by many readers. It makes him judge and jury of what is GOOD, what is the right and proper morality. Of course, using the blood of children as a youth serum has to be bad, but it is still a potentially dangerous idea. And yet, The Doctor has had to do that many times in the TV series. He has to judge what it right and wrong. Classically, his dilemma about whether to destroy the Dalek embryos in Genesis of the Daleks is one of those times. A less successful one was the Gelth in the Unquiet Dead, who fooled him into believing they WERE good. This is an example of Chrístõ doing what The Doctor has done all through his life, making the decisions that nobody else is qualified to do.

http://en.wikipedia.org/wiki/Elizabeth_B%C3%A1thory