So about this time last year I pitched the idea of vertical geomorphs to the carto-dungeon-graphic portion of the OSR community. The basic idea was to take all those side-view maps that show the relationships between levels in a dungeon and actually use them like you do the level maps. The geomorph artists came through in spades, so I decided to make a vertical map the centerpiece of my new dungeon. Here are the parts that have been pretty well explored.
Because I actually wanted to use this map I decided that all the initially available entrances to the dungeon would lead onto this map, rather than directly to one of the horizontal (i.e. 'normal') levels. Later one party excavated the Great Heap in the center of the courtyard of Castle Dundagel, opening up access directly to a medusa-infested Level 2. But the majority of visitors to these dungeons have spent part (or all!) of each session slogging through this map.
This behavior amused me greatly, because I originally envisioned the vertical map as sort of an underground Wilderness Area, where random encounters aren't tied to any sliding difficulty level. Hence the vicious metallic spiders (Dave Hargrave critters, in fact) and the vampires. I honestly expected the players to spend less time scouting the vertical access once some decent routes to various upper levels were discovered.
But I guess I didn't count on the twin fascinations caused by the Last Dragon in England and the Great Spiral Stairs. When first assembling the geomorphs to form the vertical level (The above map is not a product of the random generation of Dave's Mapper. When first assembling this thing the vertical randomizer wasn't fully implemented yet, so I stiched this thing together in MS Paint.) I decided on two things: 1) I wanted to use four "top border" geomorphs that had towers on them, to give a simple four-towered castle. 2) I wanted to see how deep of a spiral staircase I could get away with.
The latter was important to me because I wanted to give a quickie way for high level or daring low level characters to get down to deep levels. This has come up in threads at places like Dragonsfoot and OD&D Discussion. Don't just connect level 1 to level 2 to level 3. Give the players some options so they can influence how big of a challenge they'll be facing. The Great Spiral Staircase gives direct access to level 8 as well as a chance at finding the lair of the Last Dragon, labeled '11b' at the bottom of the map. The two holes leading off the bottom of the map are the known Hellmouths. One of them is under a trapdoor. So far no PC has fallen to Hell.
One other factor I hadn't fully counted on: players just love climbing shit. Seriously, most of my players love spelunking around this map, pounding in spikes, lowering ropes, dropping torches to see how far they go. In some sessions the sheer joy of exploration really seemed to trump any concern for finding loot.