The porticullis to the next area is opened by a lever hidden behind the throne, so use that to continue further. You'll find some stairs leading into another set of entombment rooms, which is where the real complications begin.
As you crest the stairs, you'll come into view of a pair of invisible ghosts way off to your right two rooms over. If you're sneaky, you're fine, but if not they'll alert the draugr here and charge you- one of the ghosts is a spellcaster, and that makes this all the more dangerous, as the computer virtually never accidentally hits the wrong target with its magic.
If you're especially sly, you'll creep into the first room and then hang a quick right to step into an unobserved room. Don't touch anything just yet- instead turn left and sidestep into the doorway to the fourth round room- a draugr is standing in a creche to your right from the doorway, and if you're still hidden you can sneak attack it and kill it without alerting the ghosts. From there, you can hang a left and sidle up to the ghosts to pickpocket them and sneak attack from point-blank range.
If you take this encounter headlong, make sure you take out the ghosts first- the spellcaster makes the fight way more dangerous than it needs to be (he's the ghost on the right) and the draugr really doesn't do more damage than the ghosts, but will take more hits to kill off.
Once the lot are dead, head to the room where the draugr was waiting and snag a trinket or two off the table, then unlock the door here to access a chest and an abandoned shield.
With that done, turn your attention to the pillar that the four round rooms are centered on. You'll find that one side of it has a lever, and the other three have rotatable pillars. Check the ceiling of each room for its symbol, then turn the pillar in that room so that the symbol matches. Once these are set, come around and pull the lever to open the next door. If you fail, you'll be barraged with poison darts, so try not to get it wrong too many times too quickly.
