The Yagura (watchtower) Gate of San-no-maru is part of the remains of the Saiki Domain's Tsuruya Castle, located in Saiki, Oita Prefecture.

The Saiki Domain was founded when the first domain lord, Takamasa Mori, moved from his former land of Hita, Bungo-nokuni, in 1601 (Keicho 6), and built the mountain castle called Togamure Castle. However, due to the inconvenience of the castle's location, the castle was moved to Hachiman-yama and named Tsuruya Castle. This is the castle's present site at the mouth of the Bansho River.

In its former state, the castle consisted of the Honmaru (main fortress, including the castle tower) built in a trilaminar structure; the Ninomaru (secondary fortress); the Nishidemaru (western fortress); and the Kitademaru (northern fortress). The castle tower was destroyed by fire following a lightning-strike. In 1637 (Kanei 14), the 3rd domain lord, Takanao, constructed the San-no-maru (third fortress) at the base of the mountain. The San-no-maru became the domain's administrative headquarters later on.

The Yagura Gate was constructed at the same time as San-no-maru. The Yagura Gate was reconstructed in 1726 (Kyoho 11) and 1832 (Tenpo 3). The gate's upper part is a yagura (watch tower) and is made using the 'irimoya' structure, with 'honkawabuki' roofs, and 'kabai' roof tiles. The lower half is the Katakuguri Gate.

Today, the Yagura Gate still reflects the Edo period, along with the stone walls of Tsuruya Castle and the later San-no-maru Palace.