In Fable 3 for the Xbox 360, there are four gold keys you can collect that will open any of the hidden gold key doors in Albion. Here are descriptions of where you can find the different gold keys in the game:
Driftwood Gold Key
You can access Driftwood after completing the Restoration quest in Millfields. The first thing you need to do in Driftwood is complete three quests to gain access to the coastal islands. The quests are called An Island Getaway, Giftwood for Driftwood, and Pest Control. Once these are done, it should open up a bridge to a new island. Go accross the bridge and you’ll see a flit switch you can hit. Keep hitting it and it should move around the island. Keep following it and it should lead you to a teleporter at the top that will take you to the gold key.
Mistpeak Valley Gold Key
On the map of Mistpeak Valley, you’ll see three cave entrances for Chillbreath Cavern. Two are near the demon door location, and one is off to the side. This side cave is the one you want. To get to this cave, take the path up hill from the lake on the way to Brightwall Village, and take a left on the path before you get there. This should lead you to a cave entrance. Go through the cave and keep heading on the path going up hill. If you went the right way you should find an exit, and find some ruins over a short hill where the key is located.
Aurora – Shifting Sands Gold Key
You’ll need access to the Aurora area to get this gold key. First, complete the A Key to a Greater Key quest in the city, which will be available after the primary story is complete. Once you complete the quest, go back to the quest giver and you can buy a key for 4,000 gold. Travel to Shifting Sands and cross the sand dunes to the other side. You’ll come to a cliff side, so follow the cliff left until you find a path near some ruins. Near the ruins you should find a door that you can unlock using the key you purchased. Inside you’ll have to fight your way to the gold key.
Aurora – The Veiled Path Gold Key
Near the stairs at the far end of The Veiled Path you’ll find an entrance to The Enigma. Inside, look for a flit switch above the Enigma door to open it. In the next room, use the fireball gauntlet to light the torches. You should see some colored floor panels and some flames on the wall that outline the order that you have to step on the tiles. Step on the tiles in this order to open the next door: yellow, blue, yellow, red, blue, red, blue, red. Once the door opens, you’ll find the gold key inside.
Hidden Sanctuary Gold Key
There is also a special hidden gold key and chest to go with it in the treasure room of the Sanctuary. You’ll see the key in the treasure room in a cubby hold near the ceiling. You’ll need to gain around 5 million gold to stand on in order to reach the key. Once you get the key, you’ll need to either spend all of your gold, or transfer all of it to the castle treasury once you’re the ruler of Albion in order to open the chest at the bottom of your gold pile.