Karthwasten is a small mining town on the road between Markarth and Dragon Bridge. As you enter, you'll see the inhabitants arguing with Silver-Blood mercenaries over the rights to the nearby mines, which you can help resolve in your own way. Other than that mere distraction and the nearby mines, this town has little else to offer adventurers.
Major City? No, no loading required.
Major Shops: None
Major Quests: None
o Cooking Pot (Inside Karthwasten Hall)
o Grindstone (Outside Miner's Barracks)
o Smelter (Outside Sanuarach Mine) (Outside Fenn's Gulch Mine)
o Tanning Rack (Outside Karthwasten Hall)
o Ainethach (Male Breton, must do the quest "Convince Atar to leave Sauranach Mine" and side with him first).
Convince Atar to leave Sauranach Mine
You can get this quest after seeing the argument after arriving in town and then talking to Ainethach. Offer to help him with his problem to get this quest. Atar is inside Sauranach mine with several mercenaries. You can try to persuade him (I failed at speech level 37) or just bribe him to get him out. OR you could just attack and kill all of the mercenaries, which if you ask me is always a fun idea. There are four in total, and when you're done you can loot them and report back to Ainethach for a 600 Gole reward. You'll also be able to marry him, if you wish.
OR you can work WITH Atar and convince Ainethach to sell the mine. By going this route you'll need to bribe/persuade/intimidate Ainethach and then report back to Atar that he won't be a problem. This will get you 600 Gold for your cut and the quest will be done.