Radiosonde Castle is a mysterious piece of landmass floating in high atmosphere and is made in the image of Star of Bethlehem. Its name comes from an observation equipment from Academy City called, "Mesosphere Radiosonde," because the two can float in the Mesosphere at 52 km above sea level.
A cross-shaped construction with measurements of length and width about twenty kilometers. The mass and density of the Castle is equivalent to an asteroid capable of bringing an ice age should it cause an impact with the Earth. 200 to 300 metal spheres are on the bottom of the Castle making it float. 
It is used by a magical organization that Leivinia Birdway refers to as "They" or "Them," later revealed to be GREMLIN, to locate and chase after the Imagine Breaker of Kamijou Touma. "They" knew that Touma would eventually return to Academy City for even just briefly and that someone from either the Magic or Science side would attempt to safely land the Castle in the ocean. In order to do so someone would get on the Castle and have it break into much smaller pieces until it no longer possessed a global threat. Since the Castle is so large the pieces would eventually become bombardments dropping onto the city. 
It is surveyed by the members of the Anglican Church who then decided that Kanzaki Kaori would board it with a rocket and then safely land it, only to be countered by Mjölnir who tries to accelerate its course into crashing onto Academy City. 
Uses Feng Shui as a means of tracking the Imagine Breaker, by observing the repairing of "altered" ley lines that are destroyed by Imagine Breaker.