Okay, this is srtrange. I'm looking for the Necromancer's Amulet, and I went to Fort Ontus. Once there, I walk all the way around it and don't see a door or any other way to get in. The local map shows a doorwar to enter it, but it's just a sheer wall.

How do I get INTO this place?

http://www.uesp.net/wiki/Oblivion:Fort_Ontus

I'm afraid that didn't help. The door maker seems to be inside the wall, that's true, but their is no door in the south wall of this castle. I've walked all the way around it and there's no way in. I'm stumped.

Did you try going upstairs in the little courtyard? Go through the archway shown in that picture. If the entrance isn't in the little courtyard at ground level, try going up the stairs. I can't remember if Fort Ontus is one, but there are some forts where the entrance isn't at ground level.

Okay, I got it. I walked all the way around multiple times and found no entrances, then fast-traveled to town to sell stuff and fast-traveled back to the fort... and suddenly there was an opening in the wall. So I got this one... finally.

It's locked until you start the quest.