megical door

 Have you ever heard about dwarf doors?

The Dwarves were particularly adept at creating doorways that would fit in with their surroundings and become so well-hidden that even the people who had made1 them were unable to locate them once their identity had been discovered. From door to door, the methods for locating and opening Dwarf-doors varied, although they were frequently magical in origin.

We have two Dwarf-door examples, and they both function very differently. The West-gate of Moria responded to a specific magical invocation, while the concealed entrance on the hillside of Erebor remained hidden until it was struck by a beam of sunlight on a particular date.

The methods for opening the Dwarf-doors also varied. On Erebor, the hidden door could be unlocked with just a key once it had been revealed. The Doors of Durin, which were more magical in nature, would only open when the word "mellon," which is another term for "friend," was pronounced.

For those inside, at least in the case of Khazad-dûm, Dwarf-doors were much simpler to use: a simple push would cause them to open. However, these secrets kept Dwarf-doors concealed from and closed to outsiders. Since Thrór and Thráin were able to escape Smaug's attack through the door on Erebor, it must have been possible to open it from the inside. However, the door on Erebor appears to have been more secure.