Althea

Althea (アルシア Arushia, lit "Alcian") is the home city of the civilian and ruled by Charles De Bergt Althea The 11th. The city was made after the end of Abandonment War in 300 years ago and the calendar was named after the city. The main base of Althea Royal Guard and Special Service was located somewhere in the city

It's later revealed that Althea is actually an giant sky-island created by the instructors of 13th Special Military Forces School with uses Althing.