Reached from the Dungeon Entrance, you can reach the Treasury from here and, later, Level 2 - the Basement and beyond. In fact, it turns into a useful shortcut. It is also a route back from the Prison using two teleports in a hidden area in the bottom right. You arrive at C13, then go through D14 to reach M13 (and vice versa to reach the Prison).
You'll learn from NPCs that you can acquire new skills in the dungeon. You'll also be told that under Cleowyn's Palace is a maze infested with thieves. They seem the only ones who know how to get through the thing. An orc barbarian will tell you that polar bears and ice lions infest the lower parts of the dungeon and a troll ranger warns you to take heed of the "Keep Out" sign at the end of the corridor and to look out for traps.
Several doors need a lockpick to open them, either green or, better, blue. The central area cannot be reached until you find a teleport at the far east side, G15. Just through the teleport, some dire wolves guard a hollowed nunchukas which will give you some extra cash unless you can use it.
A square corridor around this section has a few rooms off it. One contains some elf barbarians (C5) which often carry worthwhile treasure - I picked up some good magical swords and armour here. A good lockpick is needed for one northern door and another in the south (K3 and C6).