Three stops east on the Tozai Line from Takebashi, where Palaceside Building stands, brings you to Kayabacho Station and Kabuto-cho—the formal address is Nihonbashi Kabuto-cho, Chuo Ward, Japan's Wall Street, centered on the Tokyo Stock Exchange. Opposite it, along the Nihonbashi River, stands a small shrine you may know: Kabuto Shrine. Narrow, with no resident priest, what catches the eye is "Kabuto Rock" (originally Kabuto Mound) on the grounds—the source of both shrine and place names.
The leading theory is that Minamoto no Yoshiie, known as Hachiman Taro (great-great-grandfather of Yoritomo), placed his helmet on a rock and prayed for victory during the Zenkunen War (1050s). But there is also a Masakado legend. In 940, on the road home after Fujiwara no Hidesato slew Masakado and took his head, he dropped his helmet here, buried it, and built a mound for memorial rites—the head was paraded to the capital but flew to heaven and fell at Masakado's mound in Otemachi, linking the legends.
In the Edo period, a Yoroi Shrine honoring Masakado stood nearby (separate from the同名 shrine in Shinjuku). In Meiji, shrine and mound merged to form Kabuto Shrine. Through land released to the Mitsui zaibatsu, ties to Masakado and Yoshiie faded; Ukanomitama-no-Mikoto (Inari) became chief deity, and after the TSE opened in Meiji 11 (1878), it became the "guardian of stocks."
"Kabuto Shrine" exists elsewhere too—for example, a shrine in Noshiro, Akita, has legends tied to the Oshu Fujiwara clan. Are there interesting tales at shrines near you?