(I think all the stairs are "uniform" length in step height. So I've shown the length of the stairs in the maps, I believe... these stairs only span 10'. The ones east of you and parallel start earlier but go down at least 10' (your torchlight didn't show where they ended). The stairs to the south and running west-east go down for a long way, I think at least 30'... so you have to take the info and interpret it the best you can.
I will say this, now that I think about it: the palace is at least two stories *above* ground and you are on the ground floor)
Si'Nat opens the door and finds...
Another vast room whose edges can't be seen by the torchlight at the door. However, the remains of bunks, blankets, wooden tables, and stools litter the floor in broken pieces. Five corpses litter the floor near the center. Two are human, dressed like palace guards. The other corpses are of a small human-like race with grotesque faces and livid gray skin.