In advance: Info regarding the upper level of the platform on top of the pillar and whyt lies south of it.
The upper level lies inbetween 8, 3, and 6 on the map, one can gain entrance via a ramp in area No. 3. Sheiriya spotted the people she mentioned in area 4 & 5, though as long as no-one of the group is actually up there, you do not know their exact positions. Given that the cold ball caused some trouble, some may have changed position by now ... or never will change anything again.
Area 1 is that of a stone sarcophagus, surrounded by small stone altars or the like. These days, it is apparently used as a table or something. Area 2 is a storage room.
The two skeletized guardians Sheiriya mentioned at the southern side of the bridge occupied the two southern exits to the Underdark (Area 1),
The upper platform measures some 120 feet in length and is some 60 feet wide, though, as you can see, only the tunnels and rooms are actually accessible. The Bypass on the pillar's right / East is some 150 feet long, so it takes some time to move around the pillar.
As soon as anyone is up there, I will expand on the folk up there.