Once you are the Ruler of Albion, enter the treasure room in The Sanctuary (your pause menu) and look up and you should be able to spot a Gold Key. There are five chests in the area beyond called The Prism with the various pieces of the complete Highway Man Costume. After taking on the wave of Hollow Men, stairs and platforms will align to lead to the Gold Key Door - starting just past the central platform on the thin bridge. It will float down below to the large room where you first encountered Hollow Men and received your first gun.

Here is a melee Flit Switch you can smack.

Head up the stairs and through the door on the left. Here you can make out a Flit Switch on the wall - it takes four shots from a firearm to activate it. In the latter Side Quest you can lead Saul to a secret library in the Reliquary. To get to it, you'll need to complete the Special Delivery (Dweller Camp) and The Ancient Key (Academy) Side Quests. In the Reliquary under the Academy in Brightwall Village you can find a well-hidden Gold Key Door.

Fable 3 millfields silver keys