Many people playing Hustle Castle may have notices a sign on the roof that says “Dragons Only” and has a heli pad for something that fly’s to land on. You might have to zoom in a little to see it. So far I have yet to find any dragons besides bosses. Through playing and viewing other peoples castles I haven’t seen anything that would land on the roof. This could be something that is implemented in the near future with an update. There is also a picture of a dragon in the background of the initial loading screen that could be a clue to coming into contact with one, but might be referring to a boss.