Oka Castle Guide: The Impregnable Hilltop Fortress That Inspired "Kojo no Tsuki"

Impregnable and Immortalized in Song: Discovering Oka Castle

Beyond the unfenced stone walls lies a sheer cliff. From this dizzying vantage point, a grand panorama of the Kuju Mountains unfolds. This remarkable castle stands in Taketa City, Oita Prefecture.

Oka Castle repelled the mighty Shimazu army and earned the title “impregnable.” It also inspired composer Rentaro Taki’s masterpiece “Kojo no Tsuki” (The Moon Over the Ruined Castle). This guide introduces the castle’s highlights.

Oka Castle

Oka Castle is a mountain fortress built atop a 325-meter rocky peak in Taketa City, Oita Prefecture. Known for its formidable defensive layout that earned the “impregnable” reputation, it is designated as a National Historic Site and one of Japan’s Top 100 Castles. The castle is also called “Gagyu-jo” (Reclining Ox Castle) because its shape resembles a lying ox.

325 Meters High with No Railings: Panoramic Views from Sheer Cliffs

The most striking feature of Oka Castle is its unobstructed views and dramatic stone wall scenery.

Most castles have safety railings or barriers, but Oka Castle has none. The stone walls, sky, and mountain ranges merge into one dynamic landscape—this is what makes Oka Castle special.

Below stretches a sea of green forest, with mountain ridges layering into the distance. From the Honmaru (main keep), you can see the Kuju Mountains; from the Nishinomaru (western enclosure), Mount Aso is visible. The wide-open sky offers a sense of freedom unique to mountain castles.

Look down at your feet, and the cliff edge is just beyond the stone walls. The openness comes with thrills—so be careful not to slip.

Exploring Historic Ruins Along the Walking Trail

Oka Castle has well-maintained walking paths that guide you through its historic remains.

Source: Castle Map - Oka Castle Official Website

The stone steps leading to the main gate offer close-up views of the massive stone walls and their elegant curves.

The Otemon (Main Gate) Ruins served as the castle’s formal entrance. Three indentations in the stone walls mark where crossbeams once passed through—a rare feature not found in other Japanese castles.

Sannomaru (Third Enclosure) features towering stone walls and spectacular panoramic views.

Ninomaru (Second Enclosure) has a rest house where visitors can take a break.

Inside the rest house sits a piano (from the now-closed Miyagidai Elementary School) that anyone can play. Just outside stands a statue of Rentaro Taki—perhaps your music will reach the maestro’s ears.

Honmaru (Main Keep) is the highest point of Oka Castle, offering sweeping views of the Kuju Mountains.

Honmaru also houses Tenman Shrine. In 1594, when Hidenari Nakagawa entered Oka Castle, he relocated a Tenjin shrine from the eastern part of the castle grounds and established this shrine here. Successive lords of Oka Domain worshipped here. The enshrined deity is Sugawara no Michizane, patron of learning, agriculture, and wealth.

Beyond these, you’ll find traces of the Nishinomaru (western enclosure), residences, watchtowers, and gates.

The Original Landscape of “Kojo no Tsuki”: Where Rentaro Taki Spent His Youth

In Ninomaru stands a bronze statue of Rentaro Taki, created by sculptor Fumio Asakura. The composer of the beloved song “Kojo no Tsuki” (The Moon Over the Ruined Castle) spent his youth here in Taketa.

In 1891, when his father became the head of Naoiri District, 12-year-old Rentaro moved with his family to Taketa. The ruins of Oka Castle, located behind his school, became his favorite playground. He reportedly even sat on the stone walls playing the shakuhachi flute.

In 1901, Rentaro published “Kojo no Tsuki” as a song for junior high schools. His childhood memories of playing among Oka Castle’s ruins became the inspiration for this timeless masterpiece.

The statue of Rentaro gazing into the distance with the Kuju Mountains behind him has become one of Oka Castle’s iconic images.

Note: Route 502, which runs at the foot of the castle, features “melody pavement”—when you drive over it, you’ll hear “Kojo no Tsuki” playing.

An Elevated Fortress at 325 Meters: The Impregnable Castle That Repelled the Shimazu Army

Oka Castle’s terrain made it virtually unconquerable.

Built on a 325-meter rocky mountain, Oka Castle is a natural stronghold surrounded by sheer cliffs on three sides. Positioned on a plateau formed by pyroclastic flows from Mount Aso, it was nearly impossible for attackers to approach.

The castle’s impregnability was proven during the “Hosatsu War” of 1586. When the mighty Shimazu clan’s army, aiming to unify Kyushu, besieged Oka Castle, the castle lord Chikanaka Shiga—only 18 years old at the time—successfully repelled them. This battle established Oka Castle’s reputation as an “impregnable fortress” throughout Japan.

Natural terrain combined with sophisticated castle-building techniques created this formidable defense. That is the essence of Oka Castle.

From Construction to Abandonment: 800 Years of Oka Castle History

Oka Castle’s history reaches back to the Genpei War era. Legend says it was built in 1185 by Saburo Koreyoshi Ogata to welcome Minamoto no Yoshitsune.

Later, in 1334, Sadatomo Shiga expanded the castle and named it “Oka Castle.” During the Shiga clan era, the castle earned its “impregnable” reputation by repelling the Shimazu forces in the aforementioned Hosatsu War.

In 1594, by order of Toyotomi Hideyoshi, Hidenari Nakagawa was assigned to the domain. Nakagawa spent three years on extensive renovations, transforming it into a stone-walled early modern castle. Most of the remaining stone walls date from the Nakagawa period.

The Nakagawa clan ruled Oka Domain (70,000 koku) for 14 generations, from the first lord Hidenari onward. However, a great fire in 1771 destroyed most of the castle, including the Honmaru and Nishinomaru. The structures were never rebuilt, and in 1874, all remaining buildings were demolished under the Abolition of Castles Order.

In 1936, it was designated as a National Historic Site, preserved to this day.

How to Get to Oka Castle

By car, take the Oita Expressway from Oita IC via Routes 442 and 57 (approximately 1 hour), or about 17 minutes from Taketa IC on the Central Kyushu Expressway. Free parking (140 spaces) is available in front of the Oka Castle ticket booth.

For public transportation, take a taxi from JR Hohi Main Line “Bungo-Taketa Station” (about 5 minutes). Walking is not recommended due to steep slopes between the station and the castle.

From Oita City, take the JR Hohi Main Line to Bungo-Taketa Station (about 1 hour). From Kumamoto City, both the JR Hohi Main Line and highway buses are available.

Accessible for Seniors and Wheelchair Users: The Convenient Castle Shuttle Bus

Oka Castle requires climbing stone steps from the ticket booth to the main gate, making it difficult for wheelchair users and challenging for elderly visitors or those with mobility concerns.

But worry not. Oka Castle operates a shuttle bus from the parking lot at the ticket booth to the center of the castle ruins, providing access without climbing the stone walls.

The bus is about the size of a minivan, holds 9 passengers, and is wheelchair-accessible. It runs every 40 minutes starting from 10:00 am, making it easy to use.

Service is mainly on weekends, holidays, and during busy seasons. Please check the official website for the latest schedule.

[2025.9.29] Castle Shuttle Bus Service (October–December)

Climbing the stone steps to the main gate lets you experience the towering walls up close and appreciate the elegant curves of the stonework—a rare sight in Japanese castles. We recommend walking up if possible, but if circumstances require, use the shuttle bus for an enjoyable, stress-free visit to Oka Castle.

An Impregnable Fortress and Inspiration for Song: A Castle in the Sky on the Cliffs

An impregnable fortress built on sheer cliffs.

The original landscape that inspired Japan’s beloved song “Kojo no Tsuki.”

In spring, cherry blossoms bloom in full glory—a scene selected as one of Japan’s Top 100 Cherry Blossom Spots. In autumn, colorful foliage adorns the stone walls.

And the view of the Kuju Mountains from atop the stone walls truly feels like walking through a castle in the sky.

For 800 years, these stone walls have silently welcomed travelers. Visit Oka Castle, the castle in the sky perched on the cliffs.