Dwarf-doors

The Dwarves had a particular skill in making doors that would blend into their surroundings, becoming so well concealed that even those who had made¹ them could not find them again once their secret had been lost. The means of finding and opening Dwarf-doors varied from door to door, but was often magical in nature.

We have two examples of Dwarf-doors, each operating in a quite different way. The hidden entrance on the mountainside of Erebor remained hidden until it was struck by a ray of sunlight on a particular date, while the West-gate of Moria responded to a particular magical incantation.

The means of opening the Dwarf-doors also varied: on Erebor, once revealed, the hidden door could be opened using a simple key. The Doors of Durin were more magical in nature, opening only in response to the spoken word mellon, 'friend', but otherwise remaining sealed.

These secrets kept Dwarf-doors hidden from and closed to outsiders, but for those inside, at least in the case of Khazad-dûm, they were much easier to use: a simple push would cause them to open. The door on Erebor seems to have been more secure, though clearly it must have been possible to operate it from the inside (since Thrór and Thráin escaped Smaug's attack through it).