England-Londinium Fortress

The England-Londinium Fortress (イングランド─ロンディニウム大要塞) is a magical defensive formation extending across the countryside around London, intended to protect the capital from invasion.

Principles
The outer edge of the England-Londinium Fortress is located a short way in towards London from 88km away, with something like twisted barricades set up in the fields at irregular intervals.

In the fields inside the outer edge, have been created. Unnatural signs of flattened grass within fields have been spoken of through history, with the theorized cause changing between eras, from fairies dancing to the more recent idea of. While most crop circles are the work of pranksters and attention-seekers, there are some which are not so easily explained. This kind of crop circle is artificially recreated here in order to control the grass in the fields and create a long wall of dried grass. This can then be ignited to produce a wall of flames. There are three of these encircling walls, with the third wall located about 60km from the capital.

Final Barrier
The innermost layer of the England-Londinium Fortress is the triple-quadruple final barrier (三重四色の最結界), which manifests as a dome covering the capital, irregularly emitting red and green lights, a veil of light resembling an aurora.

Rather than a simple wall like other magical barriers, there are triple barriers of four different types (just as the United Kingdom is a complex combination of three factions and four regions) located at the exact same coordinates. The gaps in the barriers are plugged by the other barriers, creating something of a labyrinth. These gigantic walls are complexly tangled together, like a mass of cables, and constantly moving back and forth. If an intruder touches one of the walls, they will be drawn into the gap between them and crushed to death.

According to Index, the barrier gives physical repulsive force to rules permeating the land and space, and uses aspects of national or regional character which work to distance outsiders. While she would have liked to compare it to the Eastern, she reckoned that it probably originated with guild membership rules which forced members to protect the group's secrets.

Attempting to untangle one part of the barrier will only get one tangled in another part, and forcibly pulling it won't do any good. The structure has been intentionally randomized such that even its creator wouldn't be able to untangle it, and enemies cannot make fixed correspondence tables using and other similar methods. The complex multi-layered construction is such that even Imagine Breaker cannot easily break through. According to Aleister, shattering the surface layer with it would only drag Touma inside quicker.

However, due to a flaw in the barrier's basic design, while it is effective in keeping humans out as it was built to do, it cannot stop gods, even the drastically weakened Magic God Othinus. Destroying the core, located in the Tower of London, will cause the barrier to collapse.

Return to England Arc
The England-Londinium Fortress was activated during the invasion of the United Kingdom and the British Commonwealth by the Crowley's Hazards, with the final barrier being closed on Holegres Mirates's orders, cutting off the external groups defending the coastline and countryside. After landing in Dover, Aleister Crowley, together with Kamijou Touma, Accelerator, Index Librorum Prohibitorum, Othinus and Karasuma Fran, managed to bypass several of the fortress's walls by flying in a non-magical balloon.

In order to overcome the final barrier, Aleister engineered Touma's capture by the Knights so he would be sent to the Tower of London, putting him in a position to find the barrier's core and neutralize it with Imagine Breaker.