San-no-maru refers to the enclosure bordered by the Ishikawa-mon gate and the Kahoku-mon gate.
After Toshiie entered the castle, his vassals’ mansions were constructed here, but later they moved out of the castle, and a couple of guardhouses were constructed, which lasted only during the beginning of the Edo period.
On its northern and eastern sides, Kyujukken Nagaya was built as a storehouse for arrows and guns.