Abandoned Weaver's Hall - Encounter Location

Up the river from a prosperous textile producing town, there's some marvel of engineering that was abandoned years ago. Who could say why?

A quick and messy encounter, the kind I play. Make your own story from the elements, ponder what in the blazes mine could be, or trust the roll of the dice. 

Drawn by hand and edited minimally - basically just a scan of my notes.