The history of Felltower has been revealed occasionally in chunks, but mostly in drips and drabs.
Back in 2011 and 2014, I posted a bit of Felltower history.
Here are a few more bits that have been discovered since then.
Construction styles in Felltower vary greatly from area to area. Some of the sections were built within the known human habitation of the underground complex. Others predate any human habitation, and show signs of have been tunneled to rather than tunneled out. Several features of the tunnel complex are clearly not of human make, and exhibit powers not ascribed to any known spell or miracle of the Good God.
A baleful force seems to dwell within the depths of Felltower, captained (?) by slender, six-fingered beings with bejeweled armor and elegantly designed weapons. Rumors make these beings out to be elves, demons, vampires, or all three. Their origins, intentions, and goals are all obscure but clearly hostile to surface dwellers.
There is a tunnel connecting the distant valleys to the north with the complex under the castle, by which the orcs have access. Rumors have placed at least two similar external connections into Felltower. These and the gates may partly explain the bewildering variety of creatures that continue to fill its dank halls even as monsters are exterminated by delvers.
Since the beginning of the clearing of Felltower, numerous groups have made their presence known in Stericksburg and Felltower. These include cone-hatted cultists, partisans of Sterick and followers, loner wizards, glory-seekers, and gold-seeking delvers.