I found my way to this floor, but that's as far as I've been. Why, is there something somewhere else that makes it clear that it's not a castle? It looks more or less like one to me.

[Considering how loose the definition of "castle" is where she comes from. Her own castle is one of the few that looks like a reasonable person would expect one to. One that looks exactly like this wouldn't be out of place in the slightest. In fact, designs for something similar probably exist in the hands of the Anegakouji, if they haven't actually built one already.]