Need ideas for a duergar fortress

[Castlewood]

So my party is playing in a heavily homebrewed world of the Forgotten Realms 3.5 ( a lot of the powerful npcs are either deceased or have never existed.)

A little backstory is around thirty years ago magical storms wracked the Moonsea down to Cormyr destroying much of the infrastructure and stability in the region.

Anyways the party has winded up in Zhentil Keep tracking an assassin down and the trail has winded up cold for them. One of the group suggested they find some busy work until they could figure out more and have found that work in an Azer (who arrived on the material plane because of the storms.) The Azer asked the party for help getting his brethren back from a group of dark elves.)
The party ended up in the Underdark through some work and the Azer is accompanying them (causing some problems for their party in the Underdark)
Anyways they found out from a group of Duergar they interrogated that the rest of the Azer were sold to the Duergar for work, and I need some ideas for the Duergar fortress if anyone could give me some tips

The party is that of a level 5 fighter, 5 rogue, and 5th level (partially homebrewed monk) The Azer is a level 2 artisan/1 fighter any tips/ ideas would be greatly appreciated

[Tyuqa]

I would first get a feel for what Duergar would want when they build a fortress.They have Darkvision so of course there would be no light in the whole place. A quick search reveals that they are supposed to able to go invisible and enlarge themselves so I would figure that a fortress of theirs would have relatively large hallways and even larger rooms to make the most of these. they also have resistances to fire and poison so they might use a weak poison gas or fire traps.

[Castlewood]

Those are all great ideas thanks I'll see what I can do with some traps and the lighting.

[Geddy2112]

A Duregar fortress is going to be the evil equal of any Dwarven hold. Take all of the negative traits of Dwarves, turn them up to 11, and play them out in material form.

Like a good Dwarven hold, expect long halls, vaulted ceilings, and fantastic stonework. Nothing is going to be subpar, and there will literally be no stone out of place. Like their closer surface cousins, form is going to follow function. There are no pointless bas relief in the walls, or statues, or unneeded gilding. The structure's beauty and function are going to be equal. The Duregar likely want to keep non Duregar out, but such methods will be beautiful shows of force. Large, thick walls, large expansive but perfectly cut chasms, corridors measured for the size and mobility of a duregar, perfect tile floors with traps that are perfectly placed and hard for non Duregar to see.

The fortress should be well planned and laid out-nothing was built on a whim, nor there by chance. It will have a logical structure. There will probably be external traps, but once the players breach the main wall I doubt the Duregar ego has a second line. Plenty of locked doors and hard to scale defenses, though.

Your party is very martial heavy-give the rogue some doors to pick, and a few traps to deal with. Guards, patrols, and a good brute force out of combat challenge(pushing, pulling, etc) should keep the fighter entertained. Duregar often have their own monastic orders dedicated to some dark art, so giving your monk a 1v1 equal or a chance to square off would be good. Alternatively, let your monk's strength of spirit show through will save checks, or allow them to matrix run on the walls, or something.