Walls of Moria
At the height of Eregion's power, a road ran out of that land towards the western Misty Mountains. At the Stair Falls, it led over a small cliff, and then down a shallow valley running eastward, and led towards a wall of immense, unclimbable cliffs: the Walls of Moria.
At the road's end was the West-gate of Moria, which at that time would have been kept open most of the time, guarded by doorwardens. In the middle of the Second Age, the War of the Elves and Sauron saw Eregion laid waste, and the West-gate was shut. By the craft of its makers, the doorway blended into the Walls of Moria, and could only be opened again by one who knew the password.