The stair from the courtyard goes down a long way. A hundred cut chambers under the rock, doors 201 to 300, and no daylight in any of them. Same rules: open the door, search what is down here, and get into a cut-out in the wall when the lamps start to go.
Darkness takes the lights out of a whole chamber, so learn the shape of a room while you still can. At door 289 something is pacing between the cabinets. Get the five keys and get out. It will not forget you, and at door 291 it is waiting in the doorway behind you.
Keyboard and mouse required. Sound on is better.