I've been thinking more about the structure of my megadungeon, and the implications that a central shaft driving down practically to the lowest levels of the dungeon. Players will be able to, from the start, descend the entire way down to the very depths of my as yet unnamed megadungeon. It'll be a very exposed descent, but they could theoretically do it. On the other hand, it'll be hard for a player to pass by all of the portals and doorways in between the surface and the lower levels!
Monsters of the megadungeon. There will be a fair amount of vermin in the upper levels - lots of unintelligent critters. Kobolds will be pervasive throughout the upper levels, though never in great numbers. They've discovered that small tribes survive much better than large ones. They attract much less attention! Encounters with them will probably be with mining/scouting/salvage groups. They'll often be attracted to the sound of combat, yet not reveal themselves until after the fighting is over, and they can pick through whats left, and if the victors appear suitably weakened, then they'll attack.
One thing I don't think I've ever done in all my years of gaming is to create random monster charts. I've used them, but I've never created my own. Now I'm going to have to create some for the unique ecology of my megadungeon! The way I figure it, the shaft is going to require it's own wandering monster table, possibly 2 of them, one for the upper half, one for a lower half. The first couple of levels from the surface can mostly all share a chart (with a few exceptions I expect).
I continued to work on my dungeon this weekend. First up is the vault sublevel below the dwarven hall.
Next up I've got a section of mine. I think this is from the abandoned hobgoblin area. You can see the finished room where the miners stay after their shift. Being hobgoblins I figure they probably used slaves to do the worst jobs. In the "How to Host a Dungeon" game that I played, the hobgoblins encountered the cave of fate, ending their existence. I haven't figured out what exactly that was yet, but there are a number of possibilities ranging from the mundane to the exotic. Luckily I don't have to settle on the answer now.