Not much is known about the depths of the Venzia Fortress - Anyone who ends up on the island usually does so by being taken by a Harpy, and at that point they’re guaranteed to be Harpy food anyway. One Harpy in the flock, Anteria, will occasionally make mention of the ancient fortress, however, and some of her victims have been able to scribble notes and drop them for others to find. Scholars have found these notes and pieced them together, forming a vague story about a magical artifact that keeps the island afloat above the sea to the West, and the ancient home of an old Dragon who may or may not still be living there.
Anteria herself is very quiet, and acts as a guardian of sorts in the fortress, along with several other Harpies. They still leave to hunt for food, which is how these stories get spread around, but when they’re on the island, they’re in the fortress. Not even the other Harpies know what they do in there - Not that they care in the first place though.