Windsor Castle

Windsor Castle (ウィンザー城) is a stronghold in England in the county of Berkshire outside of London. It is the secondary royal residence of the British Royal Family. It is mostly used for parties, while Buckingham Palace is used for meetings, though is powerful enough to be classified as a stronghold-level defensive barrier that is not less than Index's Walking Church.

Principles
According to Tsuchimikado Motoharu, the strength of Windsor Castle is not less than that of Index's Walking Church, and is strong enough to defend, at least partially, its inhabitants from a Grand Magic like Angel Fall, similar to Southwark Cathedral and Westminster Abbey. The castle also has magic locks that can be placed on doors, one of which is extremely high level and worthy of guarding the royal family. Queen of Honors are also employed in the castle.

Windsor Castle also contains a large room where crests are displayed like Buckingham Palace, though in the castle, it does not hold any magical meaning.

British Royal Family Arc
When Elizard's daughter, Carissa, retrieves the Curtana Original and takes control of the Knights of England and starts a coup d'état, she and along with Archbishop Laura Stuart are detained in Windsor Castle.