After opening all the doors on the upper level, south side of the MAIN HALL, go through the right door (K in the diagram).
Light a flare and save the game before sliding down the ramp.
As you slide, jump to grab the ladder above and in front of you just before the end of the ramp.
...to emerge on top of a huge statue of a lion's head. You'll hear a rumbling as a metal globe rolls down the ramp on the opposite side of the room. You're safe up here.
Take a running jump from the top of the lion's head to the long ledge on the right (west) wall. This is SECRET #3 (52/70), a large medipack and normal arrows.
- OR -
If you miss the concealed ladder and drop out of the lion's mouth onto the ground, either reload your saved game and try again, or...
...take a standing jump up the slope then flip to either side...
...to get out of the way of the rolling globe.
Fight the mechanical man that appears in the southeast corner of the room. (You can jump onto the nearby block to avoid him temporarily.)
Then use the crawling corner bug at this corner on the west wall to reach the secret on the ledge above.
...to here, like so. I have reviewed this maneuver a few times in earlier levels, so I won't go into detail here. If you're not sure how to do the crawling corner bug, check out the Useful Bugs section or some of the previous examples in the Tomb of Semerkhet and Catacombs levels.
Pick up the items and safety drop to the ground.