After using the SILVER KEY to unlock the door near the road, enter the excavation area. Run forward to the first gap in the low wall, turn right, and kill 2 more enemies...

...who jump down from the excavation walls. There are 2 levers here, on the northeast and southeast ledges. Before using the levers, turn around...

...go through the gap in the wall to the north...

...and retrieve a small medipack from the alcove in the southwest corner...

...and shotgun shells from the alcove in the northwest corner.

Return to the east side of the excavation, climb onto either wall, and take a running jump over the deep pit to one of the levers. Use it.

Then make your way across the other pit to the second lever.

When you use the second lever, the metal door in the northwest corner opens.

An assassin sneaks in from the west and climbs onto the wall.

If you're ready for him, you can kill him before he reaches you. The crossbow, with regular or poisoned arrows, works especially well since arrows slip right past his whirling blades.

When the coast is clear, jump back across the pit and head for the northwest corner and the door you just opened.

[Sphinx Complex Walkthrough]