From the wiki: “The main Hongo campus of Tokyo (Imperial) University occupies the former estate of the Maeda family, the Edo-period feudal lords [daimyo] of Kaga Province. One of the university’s best-known landmarks, Akamon [red Gate], is a relic of this era.
“Akamon was erected in 1827 by Maeda Nariyasu, the 12th Lord of the Kaga, to welcome Lady Yasu-hime, a daughter of the 11th shogun of the Tokugawa Shogunate, as his bride. Such gates were built for the purpose known as goshuden-mon, the feudal tradition requiring that a daimyo whose family was marrying into the Tokugawa family erect a spacious home with a red gate. Akamon was constructed in the traditional gate style called yakui-mon and painted red, as was the conventional practice when receiving a shogun’s daughter as a bride. A design featuring an arched gable with undulating bargeboards (a style called kara-hafu) together with two guard posts on the sides of the gate was a style of the highest rank and allowed only for the most prosperous daimyo.
“Akamon became the university’s venerated entrance in 1903. At the end of the Meiji period, Akamon was dismantled and reconstructed in its current location, 15 meters west of where it originally stood (see Whole View of Tokyo Imperial University, c. 1930 for an aerial photo of its present-day location) – replaced by a brick and concrete reminiscent of the original Akamon (see below). Akamon is the only goshuden-mon still in existence and was already registered as a National Treasure before World War II. It has since been designated an Important Cultural Property. The gate was extensively repaired in 1961 and still stands today, watching over the comings and goings of the students.”