A castle game oshirogo (御城碁), in relation to high-level go played in Japan during the Edo period, was an official match played by representatives of the four go houses in the castle of the shōgun. In its original intention, the two players would play in the shōgun's presence.
With the passage of the years, this became a formality: the players would replay a game that had already been played, and the shōgun would often be represented by an official, rather than attend himself. The games themselves were, though, bitterly contested, since the castle games had a major effect on the prestige of the four houses. Throughout the Tokugawa shogunate there was an ongoing struggle to take control of the official positions of Meijin and godokoro.