The basic idea is that a group of monks created a magical prison called Solitude. This artifact was composed of a spire and many individual cells, in which the monks would secure valuable items for powerful people. Think something kinda like Fort Knox for whoever could pay the highest. Over time the group degenerated and eventually the location was forgotten. Which is where the PCs come in, obviously.
I imagined the monks as a sort of Clunianc/Carthusian-esqe order that lived most of their time in their own quarters tending their own private garden (the “Cell” as it is known) and only coming together to pray and eat. I cribbed the monastery map from the great Charterhouses of Clermont and Nuremburg. I then just attached the prison Solitude to the construction and volia, the above ground level. The below ground levels were conceived as a mix of carved work and natural caves (which the keepers used for mushrooming, of course!).
I’m very proud of The Lost Keys of Solitude, there’s the above ground challenges of gnolls led by a pair of spriggan who’ve just managed to kill a tyrannosaur they foolishly released from the prison Solitude, a nest of giant bees, and of course a spirit of a slain keeper. Solitude itself allowed for full freedom regarding what could be in each cell, from valued treasure, to an avatar of famine, a water elemental prince, and an ancient red dragon.
Underground, the PCs can befriend a race of intelligent rats (called Mus Maxumus) and help the fight their enemies (intelligent suped-up groundhogs), combat the de-evolved remaining keepers, and eventually find a pair of giant black gates beyond which they cannot travel (which of course the GM could use as a set up for further exploration).
As a bonus, there’s something like 10 new monsters and half again as many new magic items in Solitude to entertain the most-experience player. Below are some interior illustrations by John Bingham and Jeff Womack. One of the cool monsters is the Bone Sovereign, a nasty undead that can absorb skeletons into itself and grow stronger: it can also “spawn” skeletons back, effectively being its own walking party. Another is the haemovorid, a mix of fairy and stirge (*shiver*), and the last is a jellyhive, a critter that eats jellies and slimes and spits them out as a defense mechanism.