What are these megadungeons like?
Purpose of the Spires and their Megadungeons
The original purpose of the dungeons was safe mass teleportation; even in the time of the ancient (one might say epic) era of Kingdom of the Wheel, earth blocked all teleportation. To ensure that no small amount of built up dirt, kicked up by winds or movement, would interfere with the movement of troops and valuable commodities, the spire teleportation system was created.
A teleportation spire has the visible spire towering above the ground, allowing teleportation beyond the horizon. The lower half (whatever shape it becomes below the ground) drops into the earth into a large dirt-free chamber far below the surface. Around this teleport platform, there were various staging areas, corridors, security checkpoints, offices, quarters, stairs, featherfall pits and levitation tubes for the transport of the troops and commodities in and out of the underground area.
Of course, because this was underground, the dwarves of the area were involved in the construction and in some cases even linked some of their secure Underground Halls and Highways to the underground spire facility.
The Coming of Chaos
During the war with the Overking of Chaos, these underground areas were further enlarged to house wartime personnel to defend the network against the potential infiltration of the Chaos Courtiers. Because of these precautions, not all of the Spires fell to Chaos and in most cases even those that did were able to secure the auxilliary teleport chambers to allow the surviving forces of the Kingdom to mount retaliatory invasions.
However, the presence of Chaos had some effects on the size, shape, on the original layout of the underground spire complexes. Furthermore, the fall of the Overking of Chaos did not signal the end of all of his Chaos Courtiers. Most were greatly diminished in power and had to sacrifice much of their power to retain some semblance of physical existence in Enigmundia's reality. A number were enslaved or had their powers stripped and transformed by the devils and demons and lesser gods of the realm. The remaining few ensconced themselves in portions of the Spire Complexes, warping the rules of reality in a -- to them -- pitifully small area of influence to survive. It's rumored, that other Chaos Courtiers did the same to other parts of the Kingdom of the Wheel and its surrounding lands (some mountains, forests, and portions of the Dwarven Underkingdoms).
Behind the DM's Curtain
All this is a rationale for dungeons and megadungeons in the Kingdom of the Wheel, and a rationale for various patrons and industries to back megadungeoneering (particularly in the case of the Spire megadungeons) which I'll tackle in a later post.
It also allows folks interesting in using much of the online material for megadungeon creation to do so -- fitting the existence of these megadungeons into the Kingdom's backstory is pretty easy. For example, one of the 'big baddies' of Stonehell Dungeon is easily recast as a Chaos Courtier -- one whose existence can be built up with rumors and/or logs from the military archives of the bygone era.
Also, the presence of the standard goals for megadungeoneering in the Kingdom of the Wheel (map the dungeon, find the teleport areas, clear the facilities for use) is an argument for more facilities and support from the outside world for adventurers, as well as a source of fresh adventurer gossip (various failed and successful expeditions with new updates to the map).
As for a canonical map of the spires -- I don't have any. However, I'm currently considering the work of 0One for The Dungeon Under The Mountain (see the image above for a neat teaser image) as the Shadowspire megadungeon. To date (and I think they're done), there are ten levels to this interactive PDF product, and the links to each of the levels can be found below:
In addition, I'm checking various blogs for the wealth of resources on megadungeon creation, and here are the ones I've found so far... oh wait, someone's done this already.