The Siege of Tachibanayama Castle occurred in the eighth month of Tenshō 14 (1586).  The conflict was waged between the Shimazu and Ōtomo clans at Tachibanayama Castle situated atop Mount Tachibana straddling Hisayama and Shingū in the Kasuya District of Chikuzen Province in Kyūshū.

Prelude

In 1586, the Shimazu army, aiming to extinguish the Ōtomo clan, marched north to Dazaifu.  This was near Iwaya Castle and Hōmanzan Castle in the Mikasa District of Chikuzen Province.  At this time, Takahashi Jōun, the father of Tachibana Munetora (later known as Tachibana Muneshige) and a senior retainer of the Ōtomo clan, was at a lightly guarded Iwaya Castle with a garrison of 763 soldiers.  On 7/12 of Tenshō 14 (1586), the Shimazu army issued a demand for surrender, but Jōun did not accept it and instead committed to a fight to the end.  On 7/14, the Shimazu forces commenced attacks on the castle.  However, a majority of the Shimazu army hailed from other provinces and lacked an esprit de corps.  Under the command of Jōun, the Shimazu army was repeatedly repelled and lost a frightening number of soldiers.  Struggling in their attack against the castle, the Shimazu offered a settlement if Jōun would send his son to them, but, once again, Jōun refused.  A stalemate ensued for several weeks until, on 7/27, Shimazu Tadanaga himself commanded an all-out attack on the castle.  After incurring numerous casualties, the attacking forces penetrated the defenses and, finally, only an inner citadel remained where Jōun was holed up.  Jōun proceeded to climb a tall watchtower and committed seppuku.  All of the other defenders died fighting, whereupon the battle came to a dramatic end.  This is known as the Siege of Iwaya Castle.

The Shimazu lost many of their own soldiers in the attack on Iwaya and it required a period of time to reconstitute their army.   In pursuit of the aspiration of Shimazu Yoshihisa to unify Kyūshū, the Shimazu army proceeded toward their next objective – Tachibanayama Castle defended by Tachibana Munetora, the head of the Tachibana family under the command of the Ōtomo clan.  At the end of the seventh month, Munetora, at the age of twenty, holed-up in the castle to sternly resist an assault by the Shimazu army totaling 40,000 troops.

Course of events

Shimazu Tadanaga led an army of 30,000 to 40,000 men and marched toward Tachibanayama Castle.  In the middle of the eighth month, these forces set-up a camp at Takatorii Castle which was vacated at the time.  Munetora and 3,000 men were holed-up in Tachibanayama Castle.  After surrounding the castle, the Shimazu demanded the defenders to vacate.  Munetora received news that reinforcements of Toyotomi Hideyoshi (led by Kobayakawa Takakage and Kikkawa Motoharu) had arrived at Akamagaseki in connection with the campaign known as the Subjugation of Kyūshū.  This prompted him to reinforce his defenses at the castle and buy time until the arrival of the Toyotomi army.  On 8/24, Uchida Shigeie, a senior retainer of Munetora in Tachibanayama Castle, proposed to the Shimazu army that the Tachibana would accept the demand to vacate the castle.  The Shimazu responded by holding Shigeie hostage and paused their assault against the castle but this was a ploy by Munetora.  While the Shimazu army was laying siege to Tachibanayama Castle, the Mōri army under the command of Hideyoshi advanced to Kokura Castle in Buzen Province.  As a result, the Shimazu army was forced to withdraw.

Thereafter, Munetora immediately led soldiers out of Tachibanayama Castle to attack.  These forces captured Takatorii Castle, killing Hoshino Shigetane and Hoshino Shigemoto (siblings) under the command of the Shimazu.  Riding this momentum, the forces took over Hōman Castle and Iwaya Castle where Jōun took his own life.  As a result of these battles, the Shimazu army was compelled to abandon their plans to unify Kyūshū.