Among the fantasy tropes used to begin a megadungeon campaign is the abandoned monastery.
Legends tell of an abandoned monastery that was founded by an ancient order of warrior monks, who were tasked with the responsibility of containing some evil presence found beneath the site of the monastery. Over time, the monks either became complacent and abandoned their posts, or were corrupted by the evil they were charged to contain.
Now -- years, decades, or centuries later -- the evil beneath the abandoned monastery has grown to proportions significant enough to come to the attention of the players or their benefactors.
The players travel to the site to determine the threat posed by said evil presence and, if necessary, eliminate it.
The above illustration would be a neat hand-out, to present to the players as an illustration of what they see as the approach the abandoned monastery. But why use a black and white illustration, when you can hand them a photo of the actual freaking chapel!
This is the Saint-Michel d’Aiguilhe Chapel in Le Puy-en-Velay, France and was built some time around 962 A.D.
The entrance can be reached by climbing up 268 foot-high steps, carved on the exterior of the basalt extrusion that forms the mountain beneath the chapel.
Imagine having to climb that mountain every time you intended to begin your dungeon delve.
I would love to get the floorplans for this Chapel, and use it for an entrance to a megadungeon, with the dungeon proper following some endless staircase driving deep into the basalt extrusion and far beyond.
You could make the "endless staircase" the centerpiece of the dungeon architecture, with various levels hiving off, spoke-like, around a central staircase, complete with bottomless pit at its centre.