Turn about and head to the chamber where a draugr was watching you enter. You'll find another waiting in a creche to the right of the doorway, on the same wall as the doorway (and thus slightly inconvenient to reach before it awakens) and another watching you from the next room over. Take care of the both of them and then move into the next room, which is lit by a brazier.
Loot the draugr and the burial urn, then look on into the next hallway. This hall is blocked by a series of porticullis, operated jointly via a set of four levers on the walls. Have a bit of fun experimenting here- each lever operates multiple gates, and all of the gates are linked to more than one lever. You can try to puzzle it out, but it seems to work just as well to spin around pulling levers until everything's open, so long as you don't pull the same lever you just pulled at any point.
Advance past the portcullises (portculli?) to reach a set of stairs you can descend- the way down is blocked by a pile of rubble, but the stairs are thankfully much wider than the rubble is. Take care as you cross the stairway, though, as a rockfall trap will alert the draugr at the bottom of the stairs to your presence. On the upside, this prompts them to open the door for you and there's a chance you can snipe them from the stairs rather than having to face them at melee range (or you can charge in screaming and hollering. Up to you, really).
On entering the room, carefully loot it and make note of the features- a pull-chain on the left wall, a stairway under the floor grate, and a pair of levers beside the two thrones. The chain only triggers a hail of poison darts, but the levers each open their respective doorway behind the thrones. First advance through the right hand doorway, where a puzzle-key head waits in the middle of the room.
As you enter the room, a pair of draugr will burst from their sarcophagi in the tiny space beyond the far doorway- take them out before inspecting the room. You'll find that the heads are Snake, Whale, Eagle, from closest to your entry point to the one facing the twin sarcophagi. Making note of that, exit the room and pull a lever by a throne to open the other doorway.
