I just cleared out Fellglow Keep. After having finished it, I did a little reading about it and found it is a large part of the Mages Guild quest line.

Since I haven' started that quest line yet, does this still bork the quest when I finally get it?

I gotta say it really svcks that this could possibly screw up intended quest lines.

What I really want to know is should I go back to an earlier save now before I get to far along?

I don't think so. The areas that you need to go to for that quest are blocked off by collapsed ceilings if you go there without the quest, and UESP doesn't say anything about it hurting anything if you go there early.

Usually places with quests that can be broken by going there beforehand will have their doors locked and requiring a key to open.

I cleared it before getting the quest line. Had no problem at all with the quests. Everything essential respawns.

- Dave

Odd. Here is what I read over on the wiki...

• There is a bug that, when you clear out Fellglow Keep before you accept the related quest (Hitting the Books) you'll be stuck on the loading screen. (PS3,Xbox360)
  
• It seems that the above mentioned bug exists on the PC as well except that it manifests as a CTD when trying to enter the keep. (PC)

And the UESP said the same:
"If you clear out Fellglow Keep before you accept the quest Hitting the Books, during the mission game will jam when trying to go in the dungeon."

I'll argue with that. "the game will jam" is most certainly what did NOT happen with mine and I'm on a PS3.
I don't promise it won't happen, but maybe it's a little more accurate to say it "may" jam.

- Dave

Cool. I think I'll risk it and move on. This game is too huge to do things twice.

8^)