Nijubashi (in English, double bridge) is the name of bridges at the front gate of Imperial Palace. The inner bridge is "Seimon Tekkyo" (the iron bridge of front gate), whereas the outer one is "Seimon Ishibashi" (the stone bridge of front gate). The origin of the name double bridge is that the inner bridge at Edo era had double-structured, of which lower bridge increased the strength of upper bridge, so originally only the inner bridge was called Nijubashi. But now it has been changed and the official view of the Imperial Household Agency is that the origin of the name is derived from the view of two bridges. Both bridges were originally wooden bridges, and the inner bridge was build as stone bridge in 1887, while the outer bridge was built as iron bridge in 1888, using German architecture. The gate connected with Seimon Ishibashi is Ohte Mon, and the building at further inner side is Fushimi Yagura.
・・・という感じでしょうか。 では続きまして、その「伏見櫓」についても m(_ _)m
Fushimi Yagura is a small castle inside the Imperial Palace at Ohte Mon. It is told that this building was derived from Fushimi castle of Kyoto. The current building was rebuilt after collapsed at the Kanto Great Earthquake in 1925.
Kyoto Fushimi castle was originally built at Yubitsuki along Uji river by Toyotomi, Hideyoshi, and after collapsed in 1596 it was moved and rebuilt at Momoyama. The castle was attacked and fired by Ishida Mitsunari troops, then repeatedly rebuilt by Tokugawa Ieyasu.