Entered Kogoruhn for the first time yesterday, and I was wondering; what is a good level for an Altmer mage to attempt this dungeon at? I'm level nine, and I'm totally getting my butt kicked by Dagoth something-or-other and his Bonelord friend. Is this normal? Should I leave and return at a higher level?

On a related note, does anyone else think the Dunmer fortresses really look like something out of Daggerfall?

Kogoruhn is visited eventually as part of the main quest. It does have a fair amount of set level enemies (most of the Dagoth's are lvl 16+ if I remember correctly), so if you're finding it too tough loot what you can and return later to try again. It's normal in Morrowind to come across places that have enemies much stronger than you at lower levels. Typically you either have to think outside the box and be creative or come back later when you are strong enough...

I'm actually in Kogoruhn because of the main quest. Why the Ashkhan needs a pile of corprus weepings to tell me about the Third Trial is beyond me, but whatever. However, I'd have thought that a main-quest dungeon at this stage would be a bit easier (I mean, with fewer enemies that can two-shot me. I do expect a challenge though). Or is it unusual to have reached this stage in the main quest and still be level nine?

Not really. It depends on what skills you've selected (and used) and if you're concentrating on the MQ only. At level 9, you can be a wimp ( if your levels are in non damage skills ) or a beast (if everything went into weapons or offensive magic). As a level 9 mage you should have enough Alchemy to make good restore health and magicka potions and plenty of Destruction (or Mysticism if you like absorb health). If you're not a pure mage, a bound bow is good too, along with a little Marksman training to 40 or so. Also, a bound bow is very useful in the expansions when you meet some resist/reflect magic enemies.

Level 9 is a bit low-level to reach that stage of the MQ, but not drastically. I vaguely recall having tackled it at around L11 or 12, and it was tough but doable. Depending on your character and skillset, it might be anything from nothing special to next to impossible.