We just did this instance with a group of 7.
Paladin, Minstrel, Cleric, Merc, Friar (All lvl 36) and two low levels we were PLing.
No problems at all. Mobs started out orange to us, after about 1/3 they became red. Boss was purple.
So I'd put the lowest possible level for this at 36.
Mob pathing is quite random here, though. Sometimes mobs from the lower level of the pathway would add, run a loooong way to one of the stairs, run a looong way to one of the bridges and so on to finally reach us. So be wary for adds that might take a while to reach you.
Edited, Sat Jan 7 16:23:06 2006