This was the easternmost of the so-called "Twelve Garrisons of Southern Hokkaido", built on the Oshima Peninsula by the Wajin from the fourteenth century.

[3][4] Shinoridate is located some 9 kilometres (5.6 mi) to the east of the center of Hakodate, along a stretch of coast with many good natural harbours.

[1][note 1] The Hakodate City Board of Education conducted excavations and surveys of the enclosure and surrounding area between 1983 and 1985, uncovering the remains of a number of buildings, palisades, a well, artefacts made of bronze, iron, stone, and wood, celadons and white porcelain from southern China, as well as domestic Suzu, Echizen, and Seto ware.

[1][7][8] Three different intercolumnar measurements were used in the construction of the buildings, the style of the well is that found in Heian-kyō in the late Kamakura period, while many of the ceramics are typical of the early fifteenth century.

With the archaeological evidence pushing back the origins of the fort at least half a century before Koshamain, its construction can no longer be understood as an immediate response to the contingencies of 1457, and other explanations are required.