With the traps in the King's Domicile, you will get very familiar with the routes through this area. You can also reach it by teleport from the King's Domicile, arriving at F16 and returning that way after falling through a trapdoor.
You'll learn from one NPC that the only way to get out of this basement area is through the lava field. It is deadly to touch (I didn't test this out!) and its great heat will sap your strength. (Yes, you lose 5HP/step). Another says there is a short cut through the lava field. If you can find the correct key, you'll be able to use the short cut. The key will also lead you to riches. (The key needed is the Lava Key which you find after a fight at O7. This is reached by passing through a series of one-way doors and having a lot of fights on the way.)
The Lava Key opens several doors. There is a Fountain of Hot Ice to restore your health at J2 behind the door at J3. It also unlocks doors at L7 and K8. K8 leads to the short cut (the 'long cut' is via a door at M8). Visit L8 too for a lucrative fight with wyverns which is the one the NPC was talking about.
A teleport to the Level 5 Pit Bottom is at P1. To enter the bottom left corner, you'll need to use the teleport at G7 to get to A2. Several one way routes (doors and walls) round here. The bottom right area, which is large but mostly empty, can only be dropped into from the King's Domicile. One NPC tells you there is a teleport to the top somewhere round here. Another says the pillow labyrinths are deep in the dungeon. The labyrinth is the hollow part of the lava.