After picking up the small medipack in the alcove above the pool, as shown in the previous section, turn around and take a running jump...

...to grab the bottom of the stairs opposite the alcove.

Pull up, turn left, and immediately draw weapons and kill the blue-robed baddie at the top of the stairs. Climb the first flight of stairs to the landing where he fell.

Continue up a second flight of stairs. At the top, turn right...

...step out onto a ledge above the pool, turn right again, and shoot another bad guy on next landing. Continue to the top of the ramps.

Step into the room with the lever, but before using it roll...

...and deal with a third bad guy who tries to sneak up behind you.

Use the lever.

This deactivates the flame trap in the hallway below.

You can safety drop directly into that hallway via the opening on the left, but...

...Lara will lose a bit of health from the long drop. (If you prefer, you can drop into the pool instead and climb up into that hallway.) Now that you've deactivated the flames, you can pick up the large medipack and continue along the hallway and down the stairs.

[Citadel Walkthrough]