Abandoned Underground Stronghold

Publisher's blurb: Sequestered away within the folds of a mountain stands a lookout tower. Few can imagine that it leads deep underground, into a forgotten stronghold. Past the cobwebs at the entrance a small stairwell winds deeper down, beyond a hallway and into chambers even further below. The next floor is filled with ruined rooms. Rubble litters it, alongside fallen stone support beams. Even further down waits a series of chambers, one of them partially filled with water, and disjointed human bones… What was this underground stronghold once used for, and why was it abandoned?