Harmugstahl

Location
Harmugstahl is a cave/fort located north east of Markarth and west of Solitude.

Notable Figures
Upon entering the cave you'll be greeted by an Adventurer killing a Frostbite spider.

On asking the adventurer "What was going on with that spider?" He will reply:

"I don't know, but this place is full of them. Every one I've encountered has been enchanted or the like."

"Look, I'm getting out of here. Normally, spiders make my skin crawl, but magic spiders? Blech!"

The other notable figure is the cave's resident mage Kornalus Frey. It appears Kornalus has been experimenting by enchanting the frost bite spiders, many of the spider corpses can be found lying dead on tables scattered around the cave. Those that are still alive each have a magical glow; fire, ice or electricity.

Layout
The cave is on the smaller side compared to some of Skyrim's labyrinth like subterranean areas and is very easily navigated.

There is one puzzle featured in Harmugstahl, four levers controlling a metal barred/spiked passageway. There is one Adept locked door that needs to be picked. There are also two locked doors found after Kornalus Frey, both can be unlocked by using his key.

In Kornalus' room you will also find a Shrine to Julianos.

Lever Puzzle Solution
From left to right (try right to left if it doesn't work) if you number the levers 1 to 4, they should be pulled (towards you) in this order:

1, 2, 4 - (if the spikes are still present, push 2 forward again - this removed all the spikes for me)

This will open the barred passageway.

The front spikes can sometimes glitch and not move, or are invisible. Simply exit and re-enter the fort to correct it.

(doesn't always work if you leave)

Notable Loot

 * Book: A Game At Dinner (Alchemy skill book) located behind the Adept locked door.
 * Four chests with random loot.
 * Possible location for Shalidor's Insights

Kornalus Frey doesn't appear to have any static loot besides the key he carries. The Adventurer also has nothing special if you decide to kill him, his inventory is comparable to any bandit.