Seemingly not much here. You can only access the three corridors directly. One NPC tells you the object to open locked doors is on the first level. This is the Key to Cleowyn's Palace which we found in the Vestibule. Another says there are 3 secret areas in this level but they can't be entered from this corridor. True. The only way in is from the Rune Room on Level 3, which you won't be able to do for a while yet. A third says that not all traps are dangerous; she found a useful key down one.
Some advance warning from another NPC lets you know that the entrance to King Cleowyn's Apartments are guarded by a dwarf called Deldwinn on Level 3. You are warned not to try to fight him as he is enchanted and cannot be killed by mortals. An Orc Knight also warns that Cleowyn's Palace is filled with spirits of the tormented dead and to look for an asymmetry in the palace if you wish to hear the truth about the dead king.
You can reach the Vestibule from doors at the top left (O1, P2) and stairs down to the Rune Room at O16 and P15. A door at C5 is locked but translating the runes on the tapestry next to it tells you that none shall pass save one who holds the Key to Cleowyn's Palace. This allows you to reach the doors at A1 and A2 which lead to Cleowyn's Palace.