The Sky Bandit's Stronghold is the home and main base of the Capua Sky Pirate family. They have been living there since they fell from grace as nobles and lost all of their land and assets.
It was originally an ancient fortress from the time of the Zemurian Civilization, and was hypothesized to have been built prior to the Great Collapse. It had also been hypothesized by Olivier Lenheim that it was built to be a shelter of sorts for people.
During the Missing Linde Incident, all of the crew and passengers on the Linde were transported to this fortress, and kept under strict guard in one of the rooms. The hostage were later rescued by Estelle and co. and after the Sky Pirates were arrested by the army, the place was abandoned.
The structure of the fortress is rather linear and simplistic. It possesses two entrances, one facing Nebel Valley where airships can get in and another in a cliff near the entrance of the valley, that is close to Bose. Both entrances are connected by a linear path with individual rooms in between.