Returning to the mid-level of the stairwell gets you through the door and into a hallway that leads to the middle floor.  There’s a skeleton along the way who shouldn’t give you much trouble, and watch your step- a couple of traps lurk here, triggered by pressure plates.  Keep an eye out for the side door, there’s a bit more treasure over there- you need to pull a handle nearby to open it.

Be sure you’re ready with either a fire spell or a bow before you move on.  The next room has a much lower floor, and contains a pit full of oil- and a hanging chandelier.  If you have a long-range fire spell, toss that in and watch the skeletons burn.  If you have a bow, you can knock one of the lamps off the chandelier... and watch the skeletons burn.  Mostly just burning happens here at first.  When you try to climb the stairs to the other side, though, you get ambushed by draugr- two warriors and a frostcaster who all are rather tough- and one of the warriors is using a greatsword, making him especially dangerous.  You can use the stairway as a chokepoint, though, leading your follower up it then hopping back down and pelting the enemies at range if you feel like it.

Once you’re done with that bit, it’s on to the heavily trapped passage.  This beautiful little chain of alcoves is infested not only with draugr, but one of those nasty swinging spike gate traps- and the pressure plate is right where your follower loves to run across it, especially when you’ve just carefully walked around it and are still in its striking zone.  Very unpleasant, especially when you have to kill the draugr here in melee to get the key to the door leading onwards.

After that is the drawbridge chamber, a nice peaceful break.  There’s a waterfall and pool here with a little money in the drink and some useful stuff hidden behind the waterfall- but your main point of interest is heading up the stairs.  Here you find four of the tiny obelisks with snakes, eagles, and whales on them- and a door leading to the room where your solution lies.

A trigger panel sits in the middle of the next room- you have to stand on it and watch the four doorways for the rotating stones to reveal the sequence for the obelisks.  Once each obelisk shows the same animal as the corresponding door, you can stand on the pressure plate here to get the drawbridge lowered.  Crossing that gets you onto a platform with a little loot on it, and yet another door.