Back to the middle level - Turn having the device on your left. Jump down to the walkway below and follow it to reach the metal ring you used before in order to climb down to the lower level.

Destroying the first power mechanism (and angled support) - Ignore the ring and continue to the other side. Jump the gap and head to the tower at the other side of the walkway. Natla will start throwing fireballs at you, so be careful. Vault onto the low block and climb on the ledge at the bottom of the angled support.

Pick up the Health Potion from the left and go to the back of the support.

Stand in front of the power mechanism and press the Interact button. Lara will use the hammer and smash the mechanism.

The cut scene shows that this action breaks the support into pieces that are used as floating platforms that move around the device.

Go around the support to the front side and drop to the walkway below.

Approach the edge facing the device and when a platform passes in front of you, jump onto it.

Wait until it reaches the walkway where you started the level.

You will see the stairs you came from at the end of Yggdrasil. Jump there and go to the left.

Approach the tower and notice the Health Potion in the alcove between the two pillars on the right. This is the second stone panel you pushed on the lower level.

(Note - If there is no alcove here, it means that you did not push the second panel from the lower level, so you need to go down. Refer to the part Middle level at the beginning of the main walkthrough to see how you can climb down and push the panel.)

Perform the Chimney Jump to grab the handhold on the right pillar. Traverse around the corner...

... and climb to the next handhold. Shimmy around the corner and grab onto the small handholds. Then climb up to the next set of small handholds.

At this point, a situational adrenaline begins. Natla flies behind Lara and throws a fireball at her. Quickly jump straight up to grab the next set of small handholds in order to avoid the bolt.

Keep climbing up and jump to grab the handhold on the left.

Traverse around the corner and pull up on the small square ledge. From there, jump to grab the ledge ahead and pull up.