This was a corridor called Matsu-no-roka that had connected the Ohiroma and the Shiro-syoin. This corridor was famous in the story of a bloodshed incident. The story was called Cyushin-gura, and was based on the incident that actually occurred on March 14th in 1701.
Asano-takuminokami who was a daimyo of the Akou domain had his honor damaged by the plot of Kira-kouzukenosuke who was a chief of protocol at Edo castle. Takuminokami who became impossible to put up with a repeated plot cut the face of Kouzukenosuke with his sword near this corridor. Using a sword in Edo castle was forbidden. Then, the Shogun ordered Takuminokami to do Hara-kiri (disembowel). Akou domain was collapsed. The retainers of the Ako domain attacked and killed Kouzukenosuke one year after. It was December 15th in 1702.
This was the revenge to the unfair judgment by the ‚‡overnment. The Edo citizens sympathized with this act. Even now, this drama is broadcasted every December on TV.