Japan (日本, nihon) is the home country of Koei. It also tends to be the main setting for several of the company's titles, many of which do not get localized beyond Eastern Asia.
Koei has focused on the following time periods of Japanese history with their numerous franchises:
- Ancient Japan - mythological dates; era of creation
- Heian Period - particularly the years 900–1185
- Warring States - particularly the years 1500–1615
- End of Tokugawa Regime - 1853-1869
- World War II