system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How was a single Balrog able to overwhelm the entire city of Khazad Dum?

This wasn't some random Dwarven mine, it was the birthplace and capital of the entire Dwarven empire. The city had existed since before the First Age,. a grander and more powerful city than anything the Elves had ever built on Middle Earth, it's defenders were probably all decked out in Mithril, yet it took a single Balrog to completely decimate the city and force its evacuation. Even during the First Age, it took several Balrogs and entire armies to take cities ie. The Fall of Gondolin.
gpt
Surprise, closed spaces, and a crapton of fire. A mithril shirt will protect you from a spear thrown at your chest, but not from being burned to death. Moria was a mine, not a fortress, and any defenses in place were designed to keep enemies out, not protect from ones on the inside. When they woke up the Balrog, they weren't able to get behind defenses because there were none for them to get behind, the Balrog was already inside them. In the First Age, it took several Balrogs and an army to take a city *that was ready and waiting to fight them*. The Dwarves of Khazad Dum were not prepared in the slightest to wake up an extremely powerful and evil Maiar.