The Totem

You are supposed to be about 14th level before this quest kicks in, but I've had it launch as early as 10th or 11th level. The quest itself isn't too terribly difficult; it's the after-effects that can get a little hairy. You'll receive another letter from Brisienna asking you to meet her in a random tavern somewhere in Wayrest province. Don't ignore this letter. The mission? Retrieve the Totem of Tiber Septim (a.k.a. "The Magical Dingus of Ultimate Power") from the treasury at Daggerfall Castle. I didn't mark it on the maps, but the red squares are critters (hopefully you've figured that out by now).
You enter through the front door, just like usual. The entrance to the dungeon is the door on Gothryd's side of the throne room and it will be locked. Kind of hard to get lost at this point -- keep going until you get to the big room with the platforms and pillars. Levitate over to the big platform in the middle and go out the north door.
Go west and follow the ledge around to a passageway with a critter in it. You go through the first door on the north.
This is the treasury room. There are Knights and Barbarians scattered around the upper levels of the room. When you get to the bottom, they will be Battlemages and Spellswords (see last map for locations).
After disposing of the opposition, jump down into the big hole in the middle and follow the east passage on the south wall. You'll go underwater and run into an Atronach at the bottom. Turn the wheel and come back up. The vault is now lowered (blocking your escape route -- ooops!). Now use the west passage on the south wall. There are three chains in the back corner. The right one casts a water-breathing spell. The left one teleports you above the vault, but somewhere in mid-air. The middle chain will teleport you to a ledge near the vault.
On the top of the vault is a lever. Pull it and some blue bars will move up, opening the chamber with the Totem (it's the Papoose-looking thing on the pedestal). The treasure in the vaults is nothing to rave about, so look for the other triggers if you want, but it's not worth the effort.

Although it took a few levitates to get to the Totem, getting back can be done with a few running jumps. If you didn't take care of the Knights and Battlemages in the Throne Room, you'll probably have to deal with them on the way out. Once you're out of Castle Daggerfall, the real fun begins.