Caislean Dubhcoille (literally, Castle Blackwood) is a residential castle situated in the forests of Akibimi-san, specifically within the territory claimed by its designer, Mirabelle O'Shea, known as the Dubhcoille Forest. It is home to Mirabelle, Juliet, Ichiko Suzuki, and most recently, Meichen Long, as well as an unknown but large number of pixies who seem to serve as a behind-the-scenes staff. Among the largest known homes on the island, the establishment welcomes visitors at virtually any time of the day, and even offers guest rooms where one might stay the night free of charge. Being a home rather than an inn, however, the castle has several rules that visitors are expected to follow, which are viewable in the castle foyer.
The castle's proper interior is somewhat less of a royal hall and more of a residential manor. This is not to say that it isn't finely decorated or furnished, however; Mirabelle hand-selected every single detail about most of the castle interior's public rooms to be certain each was of fine taste. Some rooms, however, were designed by Juliet: the dining room, kitchen, garrison, and aviary in particular; she was only as meticulous in selection of decor for the dining hall, and focused instead upon utility and convenience for the kitchen.
Among the rooms in the castle:
- The ballroom for social events,
- A conference room for meetings, themed after a medieval throne room,
- A library where one can relax and read; also features Mirabelle's thesis on Ravensport,
- A grand dining hall, with a fully functional kitchen ready for virtually any culinary situation,
- A dungeon, which instead of being a place of detention, serves as a tabletop/board game room,
- Plush and comfortable guest rooms,
- A very large bathroom with running, heated water and accessories,
- A rooftop sitting area, providing a wonderful view of the island at nearly any time of day,
- A sprawling garden where flowers and herbs are grown, as well as a massive area for picnics and relaxation,
- A greenhouse within the garden, where vegetables, herbs, and some flowers are grown, also where the rare Huang pepper is cultivated,
- A garrison, whose purpose is merely to provide access to the towers,
- A cellar that holds the castle stores, including grains, preserves, and wines,
- An aviary in the East tower, providing a resting and feeding place for Yuriba's forest-dwelling birds, and
- An observatory in the West tower, with a provided telescope and series of star charts and maps.
These are not the only rooms in the castle, though all of the above are usually accessible by visitors at all times. Excess herbs from the castle stores or grown during particular seasons are often donated to the Yuriba Clinic.
Closely guarded by Mirabelle and her fiancee Asuna Asagiri of the nearby Nulari Shrine, the Dubhcoille Forest is a small but beautiful deciduous forest isolated in the middle of Akibimi-san's Maple Circle. Formerly only cared for and watched by Asuna, ownership of it was given to Mirabelle as the result of a secret pact with the spirits and dryads who dwell within, and as a result is now a personal property and charge shared between herself, Asuna, and Mirabelle's other fiancee Juliet. Three buildings are found within the wood in natural clearings:
- Nulari Shrine, the Shinto shrine and home of Asuna Asagiri, her family, and houseguests, situated near to the edge of the wood,
- Suzuran Dojo, a fully functional and multi-purpose martial arts training facility styled and themed after a Japanese dojo (with some equipment purchased from Hi-chan), and
- Caislean Dubhcoille's recreational shooting range, a target range which accommodates various low- to mid-grade ranged weaponry.
A beautiful lagoon is also to be found within the wood, with clean, cool water perfect for swimming.
The house mistresses of Caislean Dubhcoille are:
- Asuna Asagiri
Also living at the castle:
- Meichen Long (Frequent cook)
- Moe-tan (Housemaid)
- Alma Meiner
Nobody but the house mistresses are allowed in the North tower. Although they do not generally speak of the reason, it has been divulged to the town council.
The garden area is home to one of the oldest living oak trees on the island.
Caislean Dubhcoille was built to withstand even the toughest environmental hazards. During the eruptions of Inishie-san in 2012, it served as one of three fallout shelters. While Mirabelle focused her efforts on shielding the entire mountain from the volcano's wrath, refugees were shielded by wards while Meichen served food and Reiko Asagiri tended the injured.