The stairs lead down about 14 feet to area 7 on the main level.

The top step is trapped. It has a hinged lid that opens into a cavity
that contains a spring-loaded trap designed to grab and crush
any limb stuck in it (like a bear trap). A successful save vs. breath
weapon allows a character to avoid getting caught. If the save
fails, the trap catches the character by the ankle. The trap’s jaws
deal 1–4 points of damage and hold the character fast. It takes a
successful Open Doors roll to pry open the jaws and release the
trapped character. A character injured by the trap moves at 1/2
speed until the damage dealt by the trap is healed.

A hidden lever in area 17 temporarily deactivates the trap by locking
the cover in place. The guardhouse residents are in the habit
of announcing themselves as they climb the stairs so the guards in
area 17 will deactivate the trap for them. See the area 17 description
for more details.
 

>>16.