In the Circle of Magi tower, in the Senior Mage Quarters there are metal doors that cannot be picked, or opened otherwise:
What is beyond them, and how does one get inside?
What's behind them is a dungeon meant for the mage origin story. Spiders infested it and one way to get a rod of fire was to help the senior enchanter kill them. Since all of the possible wardens existed whether or not you played them, there's honestly tunnels beyond them with a couple empty creates so even if you could get in during the main story-line it wouldn't be worth it.
If I remember correctly, that door is a part of the mage origin story.
You have to go in there to clear out some spiders.
I don't think it's accessible during the main story.