Fable III Gold Doors
Gold Doors are the new Silver Chests as far as Fable III is concerned, the "mega ultimate super-secret" containers for the game's biggest prizes. Much like Demon Doors, they contain unique rewards and a small area to explore within. Unlike Silver Keys, Gold Keys are used up upon opening a Gold Door, so if you're interested in opening a Gold Door, you'll want to be careful about which one you pick.

There are 4 Gold Doors in Fable III, and they are all better-hidden than the Demon Doors. This guide will tell you where they are, how to get to them, and what you'll find within them.
Bowerstone Castle Gold Door Bowerstone Castle (The Catacombs)

The first dungeon in the game is also home to the first Gold Door. It's quite possible you saw it during the beginning of the game as you escaped the castle with Walter and Jasper, but it will be a long while before you can return to it, as you cannot come back to The Catacombs until you've become King or Queen of Albion. In order to return to The Catacombs, you'll need to make your way through Bowerstone Castle after taking ownership of it, and head out into the courtyard. The door to The Catacombs is open as it was before at the beginning of the game.

Inside, you'll find a large cave area similar to where you already are (in fact, it's in the same actual level area, unlike most Gold Doors) and will likely come under attack by a pack of level-scaled monsters. Defeat them and climb up to the top of the rock spire to claim your prize, a chest containing a full 40 Guild Seals.
Brightwood Village Gold Door Brightwood Village (The Reliquary)

You may have noticed this Gold Door the first time through The Reliquary early in the game. You must complete the quest Special Delivery, and begin the quest An Ancient Key in order to gain access to it, however. During the quest, you will be asked by Saul to open a door to a hidden library within The Reliquary. Inside this library is a Flit Switch; hit it and it will move around, so continue to pursue it and hit it with whatever type of attacks required. You'll eventually follow it out of the library, and it will activate a sequence of raised platforms heading downwards. Hold off the Hollow Men attacks and follow the platforms until you reach the Gold Door at the bottom.

This Gold Door houses a sort of mini-dungeon full of Hollow Men and several Cullis Gates. Use the Cullis Gates to teleport from place to place, opening the chests as you go (there's one chest per area). There's no real puzzle to the teleporting, however, and you'll eventually find yourself back at the first one with the entire Highwayman Suit, for both men and women.
Mourningwood Gold Door Mourningwood (The Ossuary)

One Gold Door lies inside The Ossuary, an optional dungeon containing a number of hidden collectables, and pertinent to the quest Gone But Not Forgotten. To access this Gold Door you'll have to first complete the quest Bored to Death, with the ghosts Sam & Max, and then at least begin the Gone But Not Forgotten quest, which will give you entrance to The Ossuary. Once inside, the Gold Door can be found easily by climbing the stone steps near the entrance up to the top; the Gold Door is along the walkway overlooking the area.

Inside this Gold Door lies a number of coffins and sarcophaguses, though they are inert and won't spawn any enemies. At the top of the stairs inside, you'll find a sarcophagus you can open, containing a random Legendary Weapon.
Sunset House Gold Door Sunset House

Sunset House is an optional area, but you'll likely come across it in your travels while exploring Mourningwood, as well as the Gold Door. It can be found quite easily - take the path towards the ruined mansion, then turn right once you pass through its gate. The Gold Door can be found past the main gate at the right edge of the area, buried in the forest - note that there is a Silver Key near, so if you find that, the Gold Door is quite close as well.

Compared to some of the other Gold Doors, the Sunset House's is fairly simple: it leads to a small cave with a chest in it. The chest contains one random Legendary Weapon.