The door of durin.

[Alfirin]

If you want to be really, really cynical You could also argue that doing the doors that way could have been part of a plan for future invasion (should the feud ever get that far). This revolves on four questions, 1. Do the doors ONLY respond to elvish (i.e. had Gandalf said the dwarvish word for fried, would the doors have opened?) 2. Did the doors automatically seal when closed, of did they have to be "locked" 3. Was the password the ONLY way to get the doors open, or simply a shortcut (i.e. could a large number of dwarves open and close the doors manually, like normal doors). and 4. Did the dwarves know the world to open the doors and what it meant? If the answer to 1 is "yes" and to 3 and 4 is "no", you might have the follwing scenario. Celembrimbor writes the words on the door, in Elvish, which the Dwarves do not speak or read. The Dwarves open an close the doors in a conventional manner with muscle power, from the inside like normal doors of this type (we don't really know if the opening spell works on opening the doors from the inside) Alternitively Celembrimbor tells them the word, but doen't tell them the meaning (i.e. he tells them it's a gibberish word that he made up for the locking spell, or that it means something other than it does.) You now have a situation where the secret for opening the doors is known to few, if any dwarves (who don't read or speak Elvish, but is immediately obvios to any elf who comes there and can read (like say an armed force interested in breaking in to take the fued to the Dwarven Halls) assuming they can figure out the trick (and since Gandalf's difficulty seems to arise mostly from a temporary mistranslation of one of the words, which an elf likey would not do, it would likey be childs play for them). As a protective defense against men or orcs Durins doors are likey impregnable, but against elves they would be pretty much useless.