Balrog Breaking and Entering

[Inziladun]

The Balrog that was the bane of the Dwarves in Moria, and was eventually detroyed by Gandalf, had suprised the Dwarves when they apparently delved deeper than they ought while looking for mithril.

Thus they roused from sleep a thing of terror that, flying from Thangorodrim, had lain hidden at the foundations of the earth since the coming of the Host of the West: a Balrog of Morgoth.

Appendix A

The Balrog fled from the host of the Valar and hid in Moria, or under it. However, Moria was hardly a desolate, uninhabited place when the Balrog decided upon it as a refuge. When the Dwarves came into Beleriand and made dwellings at Nogrod and Belegost, they already had cities to the east of the Blue Mountains.

Greatest of all the mansions of the Dwarves was Khazad-dûm, the Dwarrowdelf, Hadhodrond in the Elvish tongue, that was afterwards in the days of its darkness called Moria; but it was far off in the Mountains of Mist beyond the wide leagues of Eriador, and to the Eldar came but as a name and a rumour from the words of the Dwarves of the Blue Mountains.

Silm Of the Sindar

Khazad-dûm was long established by the end of the First Age, and thousands of Dwarves likely lived and worked there. There would have been comings and goings constantly, and surely the Dwarves kept a watch at the doors. How did something like a Balrog slip underground under the most populous city of the Dwarves?