Pick up the items dropped by all the monsters (including the boss); also look around for a new chest, Boatman Coin and Soul Arbiter's Scroll (you need only to approach the signs painted on the wall). After examining the area, take the corridor leading to the north door. There, use Interdiction in the summoning circle and send the Dead Lord to the pressure plate shown on the above screen.
Set Death Grip as the active ability and approach the left wall (the above screen). Wall-run on it, bouncing off at the corner. Continue running until you reach a small shelf. Before you proceed, look inside the chest.
Approach another wall and start running again. The critical thing about that wall-run is that at the very end you need to bounce off the wall, target your helper (the above screen) and pull yourself to him with Death Grip (otherwise you won't be able to reach a new shelf).
Go through the narrow passageway, finding a Book of the Dead page on the way ([The Book of the Dead] side quest). Face the interactive handhold afterwards. Jump towards it and grab onto it with Death Grip.
Wait for Death to get to the upper balcony and continue forth. Stop when you find yourself back at the main hall. Interact with the right switch (the above screen) and watch the cutscene closely - it will show you which places have been unlocked.
Start off by locating the hole which will take you back to the lower level. Approach the large gap once more, but this time turn right (towards the Dead Lord standing on the pressure plate). Stop where the screen shows and move up the wall (using Death Grip, of course).
Using the interactive handhold has unlocked access to two well-hidden items. On the lower shelf you will find a Relic of Etu-Goth (the [Lost Relics] side quest), and on the upper one a chest (the above screen).
After taking both secrets, go back down and use the interactive handhold again. This time, after grabbing onto the edge, start moving right - you will go back to the corridor on the upper balcony (the above screen).
[TIP: You can also retrace your steps with getting back to the Dead Lord, but this will take more time.]
Back on the upper balcony (the one with the switch), head towards the west door you've ignored earlier (the above screen), bumping into a few Skeletons on the way. Eliminate them and enter the new chamber.
In it, there are Skeletal Champions and Undead Prowlers with which you shouldn't have any problems. Once they're dead, examine the contents of two nearby chests - one of them has a Skeleton Key.
Backtrack to the previous room (the one with the Dead Lord) and jump down to the lower level. Your destination is now the door pictured on the above screen, which you've already passed by. Take down a new group of Skeletal Warriors, and then use the Skeleton Key to unlock access to the second tower.
Traditionally, I suggest diving for a Boatman Coin. Leave the water and start scaling the wall where shown, using Death Grip of course.
Use it again on the second handhold. Wait until Death reaches the new edge, lean back and jump towards the third handhold, pictured on the screen, which you need to catch in flight.
Be ready to catch the fourth handhold as soon as the character gets close to the new wall. Wall-run vertically, bounce off and grab onto the fifth handhold shown on the above screen.
Wait for Death to reach the new interactive edge and move left as far as possible. Lean back and jump over to the last, sixth, handhold, pictured on the screen.
The last acrobatic performance required of you comes after Death grabs onto the top interactive edge - you need to lean back and jump towards the central platform. Approach the second Lost Soul and take it.
Traditionally, this will send a new swarm of monsters after you. This time it's an Undead Stalker in the company of some Skeletal Champions. Begin by attacking the Champions, but pay attention to the Stalker's mighty and quick strikes, dodging more often than usual. Given the small size of the battlefield I also recommend area-effect attacks. Take care of the Stalker when the Champions are gone.