Du'Cragolm sits in the heart of the western mountains. Quite literally geographically as well as geologically! The majority of this metropolis is dug out of the mountain range itself. It served as the former capital of the Dwarven nation before it's fall. It now serves as a major trade hub for the entire empire.
It's halls are are incredibly spacious for being underground. Vaulted ceilings everywhere with lights of brightly glowing plants or gemstones. In many places it barely feels like you are underground. In fact, the gardens of Du'Cragolm are some of the most wondrous in the world. They are built around a warm hotspring that comes up near the surface. This is the only place outside of the gates themselves that truly see the skies. The roof of this greenhouse is enchanted with incredibly powerful runes that make it act like glass and also keeps the snow from accumulating on the top. Plants of all kinds are grown here, even those that would not normally grow well in mountainous regions. The bounty of the earth is available here to the dwarves.
From the outside, the city appears to be just a mountain like any other. The only hint it's even there is the massive stone gate house on the southern cliff face. It's the main checkpoint for anyone coming in or out of the city. That said, there are many other smaller entrances all throughout the mountain range. Many known, others secreted away by smugglers.
Due to it's impressive defenses and self sufficient nature, it was nearly impossible to siege. King Aeschere Thal only managed to defeat it by isolating the completely from the rest of their dwarven allies and collapsing the smaller tunnels where possible. Though victory by military might alone wasn't possible, enough damage was done to convince King Durnan Grimdur to sign a treaty and formally join the Ethalian Empire. In exchange for his throne, Durnan became the chairman of the new Trade Commission of Du'Cragolm and gained economic superiority in place of soverignty.