I've seen it in the distance many times, and always wondered what it's like inside. I thought it maybe could be a dungeon eventually
Since it's an airship that crashed, we're not supposed to be able to stand and wander inside.
Thus, the donjon may just be a cave under the lake and the boss is at the foot of the Agrius.
Or they'll tinker the story, saying some people inhabit the ruins and nobody have seen them until now.