how do you unlock demon doors in fable 3?

Answer 1

Different demon doors have different conditions for opening.

These can range from the one that will only open when your king, to the one that wont open untill he sees your actual child in the game which means getting married and everything that goes with it.

Answer 2

Demon doors are pretty tricky because you often have to have made certain choices years ago (character time) in order to be able to fulfill their requirements. Here's a simple guide to doing so:

http://projectego.net/fable-3/demon-doors

If you want something more in-depth, you can check this out:

http://fable.wikia.com/wiki/Demon_Doors