[Nerwen]

Originally Posted by Mnemosyne

Don't forget also about all the crazy cataclysmic stuff that was going on during the War of Wrath. Could the shaking up of the earth that must have happened with the drowning of Beleriand been enough to 1). catch the Dwarves off guard, 2). open up new and secret ways under the earth, or 3). possibly even trap the Balrog there? (Sorry, but I have a bit of a hard time thinking that the Balrog spent that much time just waiting very, very patiently for Dwarf fricasee...)

There's a note in the "Durin's Folk" section of the Appendices that suggests that rather than actually waking the Balrog, the Dwarves merely

released it from prison; it may well be that it had already been awakened by the malice of Sauron.

i.e. the Balrog was indeed trapped there by that time. It's not surprising– the area was geologically active even in the Third Age– too much for comfort, in fact– cf. The Bridge of Khazad-dûm.

Originally Posted by radagastly

To get back to the original question of the thread, why couldn't the Balrog have come in through the west-gate? For the dwarves, and for the elves of Eregion, it didn't exist until Celebrimbor arrived. (His name was on the door, wasn't it?)

Of course, this doesn't preclude some other possible entry. How about Durin's Stair? How about the top of the mountain? Of course, this implies that Balrogs can fly (wings?), but it also answers the fact that the Balrog seemed to know exactly how to find Durin's Stair (in his battle with Gandalf,) and how to get there. He's been there before. Sure seemed like it to me, at least at the time.

I refer you to my points earlier. As I said, I don't think the Balrog's hideout was connected to Moria at all until the dwarves broke into it while mining. For the sake of argument, it could have once been part of Moria proper, then sealed off by a rockfall or something– see above. However, I think it's much simpler to assume that Durin's Bane didn't deliberately choose a populous city to take its millennia-long nap in, but rather entered through some probably distant opening, travelled underground, and finally went to sleep in a cave that just happened to be near Moria.

And likely not all that close originally.

"The lodes led away north toward Caradhras, and down to darkness (...) they delved too greedily and too deep..."

–Gandalf in A Journey in the Dark.

I'd guess that at the time the Balrog took up residence, its cave or whatever was a long way from any workings of the dwarves– it was only much later that they mined deep enough to reach it.

As for how the Balrog knew Durin's Stair– well, it had plenty of time to explore Moria after its release, didn't it?