The Mosaic City is the main setting of Fate/Requiem. It has greatly changed after the large Holy Grail War 14 years ago requiring the world be rebuilt and the rising sea levels. It is comprised of the former wards of Tokyo, such as Shibuya, Shinjuku, and Akihabara. Each area is overseen by one of the Caren Series AIs.
History
Locations
Akihabara
Coastal City AkihabaraWP (臨海都市《秋葉原》?) is the setting of volume 1 and where Erice Utsumi lives. While in the old word Akihabara was once a mecca for otaku culture, in the world of Fate/Requiem that was transformed by the Holy Grail it has become a seaside resort city with many stores that sell magical trinkets. It also hosts the Grail Tournament, a fighting sport between Servants. It was overseen by Caren Fujimura before her death.
Shinjuku
Nostalgic City ShinjukuWP (懐想都市《新宿》?) is the setting of volume 2 and where Chitose Manazuru lives, as well as where Erice was raised. Its architecture is a recreation of 1920s Japan. It is overseen by Caren Himuro.
Shibuya
Shibuya (渋谷?) is where Karin lives.
Tokyo
Netherworld Tokyo is the remains of Tokyo after it was destroyed several years ago.
References