Fable III Demon Doors
It wouldn’t be Fable without Demon Doors to open, would it? The secret-hiding, riddle-spouting doors from previous games return in Fable III. Some of their requirements are steep, while others are easy to open, but the rewards, as always are some of the bigger ones in the game.

Note: if you have the Traitor's Keep DLC installed, you'll be able to gain an achievement, Set Them Free!, for opening all the Demon Doors in the game, including the new one added by Traitor's Keep.
The Brightwall Village Demon Door Brightwall Village

Brightwall's Demon Door is one of the easiest to find, located just to the right and down a short road immediately upon entering the town's front gates, past the market. To open this Demon Door, you'll need to bring your own child and perform positive expressions with him or her. The child needs to be of toddler age, not newborn, so you'll need to wait a while after conceiving, or alternately, bring an adopted child from the Bowerstone Orphanage. Within the Demon Door lies a house full of skeletons busy in merry, possibly drunken revelry, and a chest containing 3 Summon Creatures Potions and 30 Guild Seals (check the upper floor).
The City of Aurora Demon Door City of Aurora

Aurora's Demon Door can be easily missed, but isn't necessarily hidden. Follow the path that goes up the hill out of town, near the way to Shifting Sands, but take a left as you get near the exit. This Demon Door wants you to be firm in your moral commitments, which means you'll need to be either 100% good or evil (check your character status in the Sanctuary). This isn't as difficult as it sounds once you get to this point in the game, and you can quickly improve or decrease your moral standing by donating or taking money from the Royal Treasury, or eating lots of Tofu or Crunchy Chicks. Within, you'll be treated to a very attractive world reflecting your moral stance, and a chest holding a random Legendary Weapon appropriate to your good or evil nature.
The Millfields Demon Door Millfields

This Demon Door is fairly easy to find, lying roughly opposite Reaver's Manor, across the lake, behind the Timmins Towers house. To open it, you'll have to "ignore fashion sense" and put on a good deal of weight. The latter requirement is fairly easy; simply eat fattening food and drink (mutton, alcohol, pie, etc.) until your weight level is as high as it gets (check the character stats in the Sanctuary). The second requirement seems to revolve around wearing "ugly" and mismatched clothing, possibly of different colours as well. Many players have reported success by wearing the Pajama outfit of the opposite sex, and dying all the parts different colours. Inside the Demon Door is a large cave area with a chest at the end, containing 40 Guild Seals.
The Mistpeak Valley Demon Door Mistpeak Valley

This might be the first Demon Door you come across in your travels. From the lake near the Mercenary Camp, head towards the mountains, keeping to the right side - you'll find a pass through the hills with a bridge overhead, leading to a snowy area with a frozen waterfall. To open the Demon Door there, you'll need to invite a friend into your game via co-op play (either local or online). To open it, you'll need to either marry in front of the Demon Door, or perform a large number of positive expressions in front if it (hugging, dancing, etc.). Once inside, you'll find the Demon Door Retirement Home, the place all Demon Doors from the previous Fable games evidently go to, and a chest containing a random Legendary Weapon.
The Mourningwood Demon Door Mourningwood

The Demon Door in Mourningwood is fairly easy to come by; it's on the outskirts of the Eco-Warrior Village, visible in the distance. In order to open it, you'll need your melee or ranged weapon skill at level 5, and a fully upgraded Legendary Weapon equipped, which you should have once you reach the later stages of the game. The Mourningwood Demon Door holds a chest containing the full Military Suit clothing set. The environment within initially resembles the Moon; by playing with the orrery inside you can change the look of the environment to resemble several other planets in the Earth's solar system.
The Sunset House Demon Door Sunset House

Sunset House is an optional area of the game, and a little bit hidden; you'll need to enter it from Mourningwood. The Demon Door can be found quickly by sticking to the left side of the path heading up the hill. This Demon Door will only open for a monarch, so simply come back once you've become King or Queen of Albion, which, given this is necessary to complete the story, makes this perhaps the easiest Demon Door to open, if not the first you'll manage to. Inside it you'll find a chest containing 1 million gold, along with some impressive scenery.
The Ravenscar Keep Demon Door Ravenscar Keep (Traitor's Keep DLC)

You'll only be able to find this Demon Door if you've got the Traitor's Keep DLC installed. Facing the Keep's main gate, follow the bridge to the left across to the garden area. Turn left once more and you'll find a hole in the far wall leading to the shore; follow the path down and you'll come across this Demon Door who only wants to speak to a prisoner. Fair enough. You'll need to convince him you're on his side by wearing the full Prisoner Suit clothing set, the pieces of which are scattered about Ravenscar Keep and its interior. For their locations, please see our Traitor's Keep Extras page. Once you've got the outfit, put it on and the Demon Door will take you to a full-sized version of the Sanctuary map of Ravenscar Keep, complete with a giant magnifying glass hovering over it. Follow the path all the way to the end for the full Prison Guard Suit.