Today Pete, a player in the group(s) I DM, obtained "The End of the World" achievement trophy (worth 1100 xp), for his PC, 1st level Fighter Orpo, via. the party's cunning use of Levitation and Invisibility spells, reaching the uppermost edge of the mapped & stocked Fortress Eibon megadungeon. (There is still an unplumbed bottom of the mapped and stocked dungeon).
Mind you, he didn't find a staircase or corridor blocked off with "UNDER CONSTRUCTION" signs; he climbed a ladder up to the 11th (upper) level, looked around, and bailed after seeing the 8' brightly coloured snail shell in a corner next to his ladder.
Looks like I need to finish mapping and stocking the next quarter of the dungeon, which currently has 22 levels suitable for PCs of levels 1 through to 7; at this rate the complete megadungeon should have 88(!) levels and be able to handle PCs of up to 20th level.