Genpei Kassen is a historical simulation game which focuses on the war between the Genji (Minamoto clan) and the Heike (Taira clan) at the end of Japan's Heian period (12th century).

Similar to other Koei historical simulations, the player begins their game by first selecting a scenario. Scenarios include:

1. Yoritomo Lays the Foundations of Kamakura (October 1180): Kiyomori's conquests, Yoritomo's exile and rise within the Hōjō

2. Yoshinaka Seizes The Capital (July 1183): Yoshinaka's insurgent forces collide with Yoritomo, Munemori's succession of the Heike

3. Heike Restoration at the Western Sea (October 1184): Situation of the war after the Battle of Ichi-no-tani, Heike have lost property and start stranded within the west

4. Yoshitsune Slays His Brother's Adversaries (October 1185): Battle of Dan-no-ura, contention between Yoritomo and Yoshitsune in Oshu

The Heike are powerful in the first two story paths, but the Genji will have the clear advantage for the other half. The Fujiwara clan remain resourceful yet ambivalent in every scenario. Like early Nobunaga's Ambition or Romance of the Three Kingdoms, animations and duels can be turned off. Historical events are automatically triggered based on the game's date, yet the player can actively prevent them from occurring. Killing Yoritomo before Kiyomori succumbs to illness, for instance, will never trigger Kiyomori's historical death in the first story path.

The time spent in each scenario is limitless, only ending once the player has either lost or claimed victory. At any time, the player may choose to leave their side's actions to the game's AI.