Radiosonde Castle

Radiosonde Castle is struck by Mjolnir's lightning.

Radiosonde Castle (ラジオゾンデ要塞, Rajiozonde Yōsai^?) is a large airborne structure, created by Gremlin and made in the image of Star of Bethlehem.

Etymology

A radiosonde is a battery-powered instrument, carried into the atmosphere, often by weather balloon, to measure various atmospheric parameters and transmit telemetry to a ground receiver by radio.

Principles

The name Radiosonde Castle, given by Anglicans for the sake of convenience, comes from Academy City observation equipment, called "Mesosphere Radiosonde" (中間圏ラジオゾンデ, Chūkan-ken Rajiozonde^?), because the two can float in the mesosphere at 52 km above sea level.

Radiosonde Castle is a cross-shaped construction with a length and width of about twenty kilometers. It was intentionally made to resemble the Star of Bethlehem, both at a distance and up close. As well as being in the form of a giant cross, it was made to appear at a glance to be a jumbled assembly of parts from various churches and temples from around the world, each with a different style of construction, however it is actually completely new and built up from a single type of material to look like that, exposed by the even level of damage to the stones. There are staircases across the fortress's structure which are made of the same material but twisted into a form similar to construction site scaffolding, with passageways made of a narrow mesh hanging down from the ceiling on the lowest level.

Radiosonde Castle's lift is provided by a gas housed in 200 to 300 metal spheres, hanging down from the bottom of the fortress. These giant spherical tanks are a few dozen meters wide, with an estimated strength equivalent to 30mm steel.

Radiosonde Castle was created by Gremlin for the purposes of locating and chasing after Kamijou Touma, specifically his Imagine Breaker, while at the same time concealing the fact that they were searching for him, which they did through the distraction of the catastrophic threat that the fortress posed, as well as the image of a very recent threat. The size and height of the castle is such that if it dropped from the sky, it would the equivalent of an asteroid falling and its impact with the Earth would be capable of bringing about an ice age. In addition to the threat of the giant mass falling all at once, there was the threat that if it was disassembled into smaller parts to avert the global danger, those separate parts could rain down on the surface in a large-scale bombardment, causing damage even if the rest of the castle was guided to a safe landing. The relative (and likely intentional) fragility of the fortress and its lift mechanism would complicate efforts to tackle these threats.

During its deployment, Mjölnir was stationed onboard to deal with any intruders, along with buoyancy control and communication jamming measures. Gremlin had also left a message for their target, spray-painted in red on a wall in the fortress; "Welcome home, hero. From Gremlin" (おかえりなさい、ヒーロー. グレムリンより, Okaerinasai, hīrō. Guremurin yori^?).

The sub spell that Radiosonde Castle used to track Kamijou Touma (or more specifically Imagine Breaker) relied on a natural mechanism which replenishes power from the ley lines running through the planet that is neutralized by Imagine Breaker. Using the principles of feng shui, in which the locations of mountains and rivers change the flow of energy, Gremlin altered the ley lines by systemically destroying mountains and rivers somewhere, interfering with the repairing cycle so a mark that only Radiosonde Castle was automatically left behind during the process of repairing the removed power.

These magical transmitters were embedded in the ground and automatically produced at 50km intervals, given cost reasons. If the Imagine Breaker was not in range, the fortress would head for the next transmitter but if it was in range, it would be guided with more precision. The transmitters took the form of a glowing red jewel, hard enough to break a tungsten drill bit but vulnerable to Imagine Breaker.

While it allowed the fortress to accurately follow Imagine Breaker regardless of if/where its owner tried to flee, this method had a limit, after which it would not be possible to create new transmitters. During Radiosonde Castle's deployment, the final transmitter ended up placed beneath Academy City and when it was destroyed, with no new transmitters being produced, the fortress lost its ability to track its target and passed right on by without altering course. According to Leivinia Birdway, if Touma had fled without destroying the final transmitter, it may have dropped onto Academy City regardless of his location.

Chronology

Shinyaku Toaru Majutsu no Index

Homecoming Arc

Main article: Homecoming Arc

On November 5th, Radiosonde Castle had taken off from somewhere near Iceland, passing over the United Kingdom before moving onto France and Eurasia at an altitude of 52km. It was surveyed by the members of the Anglican Church, who started calling the target 'Radiosonde Castle' for convenience, with one team investigating its origins while Kanzaki Kaori's team was tasked with taking care of it. Given Radiosonde Castle's altitude meant it couldn't be reached directly through planes or rockets and the dangers of magical flight, it was decided that Kanzaki would be launched above it in an SLBM launched capsule before exiting the craft and reentering the atmosphere to land on top of the fortress, with magic merely being used for protection and steering. Having landed on Radiosonde Castle as it was passing over central Eurasia (likely over Russian territory), Kanzaki descended to the lower levels and destroyed a ballon to verify that the balloons were providing the lift before carefully reinforcing and opening holes in them with spiritual items to initiate a controlled descent.

As Radiosonde Castle was passing over Korea at an altitude of 11km, Kanzaki realized its likely target, seeing that its course would take it across Japan and right over Academy City. Immediately after this realization, her communication with the Anglican Church was jammed and Mjölnir attacked her, preventing her from interfering with the balloons while also trying to accelerate the fortress towards a crash landing onto Academy City. During the battle, a third of Radiosonde Castle was annihilated by Mjölnir's strongest attack, with not even debris left to fall onto the Kanto region, with Kanzaki averting a second shot through the use of St. Peter's anti-flight spell, dropping Mjölnir from the sky and ending the fight.

At the same time, as Radiosonde Castle was nearing Academy City, its approach was noticed by Leivinia Birdway, Kamijou Touma, Accelerator and Hamazura Shiage, the former having just given the latter three several magic-related lectures in preparation for the new enemy. With information about the fortress's tracking mechanism, Accelerator was able to remotely locate its final transmitter in the School District 7 Central Hub Transformer Station, and with help from Hamazura and a boring machine, Touma was able to reach and destroy the underground crystal moments before Radiosonde Castle arrived above the city.

Having lost its beacon and supervisor, the fortress did not fall onto the city and continued on to a controlled landing in the sea off the Boso Peninsula in Chiba prefecture. Searching for information on the fortress in the aftermath, Kanzaki came across a spray-painted message for Touma, addressed from Gremlin.

Gallery

Radiosonde Castle's design by Haimura Kiyotaka.