The claim that the erection of Sugiyama-jo Castle was by the Yamauchi Uesugi family rests on the following: the discovery of kawarake earthenware representing the characteristics of the period before the Yamauchi Uesugi clan was destroyed, the fact that Koseto style earthenware pots were made during the era of the Yamauchi Uesugi clan, that kawarake excavated from other castles have been considered to indicate a connection with the Yamauchi Uesugi clan, the lack of artefacts pointing to the era of the Gohojo clan in the 16th century, and the location of the castle itself, which as a result of research into the Kamakura kaido was found to be strategically unimportant in the era of the Gohojo clan, but was considered important as a key spot during the era of the Yamauchi Uesugi clan.
杉山城の築城を山内上杉家によるものとする根拠は、山内上杉氏滅亡以前の時代的特徴を有する土器（かわらけ）の出土、古瀬戸の甕などの遺物の編年が山内上杉氏時代に属すること、他の城から出土するかわらけの分布を検討しても山内上杉氏の城として違和感がないこと、16世紀代の後北条氏時代の遺物が出土しないこと、さらに、後北条氏の時代の鎌倉街道を調査すると杉山の地は必ずしも要衝の地ではないのに対し、逆に山内上杉氏の時代だと要衝として重要視されるという立地条件などがある。