King Gaeru came to the throne in the year 128 when his father, King Giru, died. In 132, he built a mountain fortress called Bukhan Sanseong in present-day Goyang, northwest of Seoul. Baekje fended off many invasions of the northern kingdom of Goguryeo from this fortress afterwards. This area was strategically important in terms of national defense, not only for Baekje but also for the other two ancient kingdoms on the Korean Peninsula, namely, Goguryeo and Silla. The three states constantly fought in order to secure this key point throughout the so-called Three Kingdoms period. The mountain fortress was still crucial for later Korean kingdoms. In the 11th century during an invasion of Khitans, King Hyeonjong of Goryeo Dynasty relocated the coffin of the kingdom’s founding father, Taejo, to this fortress. Also, Goryeo troops fought the Mongolian invaders in the same area in the 13th century. Today, only part of the fortress remains, and it is protected as a historical site.

During the reign of the previous king, Giru, Baekje maintained friendly relations with the neighboring kingdom of Silla in the east. But things changed during the period of King Gaeru. Silla aimed to advance to the Han River area, which was occupied by Baekje at the time. Not surprisingly conflict began to grow between the two countries. In 155, a Silla minister defected to Baekje after a failed coup. Silla requested the repatriation of the traitor, but King Gaeru refused. Finally, a war broke out between the two nations, as Silla attacked Baekje. King Gaeru closed all the castle gates and chose not to fight back. As time went by, the Silla forces ran out of food and they had to retreat.

It is said that an old tale about a diligent carpenter named Domi and his beautiful wife Arang originated from this period. After hearing the rumor about Arang’s beauty, King Gaeru ordered her to be his concubine. Arang refused and even deceived the king, the angry king banished her husband from the Kingdom. After some twists and turns, Arang managed to escape and had a dramatic reunion with her husband.

Although the name of King Gaeru appears in this tale, many suspect that the brutal king is not King Gaeru but King Gaero, the 21st ruler of the same dynasty from the fifth century. This explanation sounds more plausible, considering the notorious King Gaero who is described as an incompetent ruler. Also, it is recorded in The History of the Three Kingdoms that King Gaeru was polite and well-behaved.

The king died in 166, and his first son succeeded to the throne to become the fifth king, Chogo.