Many scenes in Demon Slayer: Kimetsu no Yaiba are inspired by real-life locations scattered across Japan. Among them, the model for Muzan Kibutsuji’s mysterious Infinity Castle is believed to be the luxurious Ookawaso Ryokan, located within the Ashinomaki Onsen resort area along the picturesque Ookawa River Valley.
This summer, the anime Demon Slayer: Infinity Castle made waves at the global box office, dazzling audiences with its breathtaking visuals and intense battles. The Infinity Castle itself is depicted as Muzan’s hidden fortress, where he gathers his Upper Rank demons to plot against the Demon Slayer Corps. Its endless corridors, layered chambers, and shifting architecture—controlled by Nakime’s haunting biwa music—create a surreal labyrinth that feels alive.
While this fortress may seem like pure fantasy, its inspiration comes from a very real destination in Fukushima Prefecture: the Ashinomaki Onsen Ookawaso Ryokan in Aizuwakamatsu City.
Ashinomaki Onsen – A Timeless Hot Spring Retreat
Ashinomaki Onsen sits within the Ookawa Hatori Natural Park. The area was first discovered more than a thousand years ago by the monk Gyoki, but its remote, rugged terrain once earned it the nickname “phantom hot spring village.”
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Today, the valley is lined with charming hot spring inns clinging to dramatic cliffs, creating an atmosphere of peace and seclusion. Highlights of the area include:
- Ashinomaki Onsen Station, famous for its stationmaster cat named Love.
- Ichii Walking Trail, a tranquil path shaded by Japanese yew trees.
- Ashi Poppo Footbath, a scenic spot where visitors can soak their feet in natural hot spring water while enjoying valley views.
A Ryokan Straight Out of Anime
Among the many ryokan in this hot spring town, Ookawaso stands out for both its elegance and its connection to Demon Slayer. Visitors often remark that stepping inside feels like entering a scene where Nakime plays her biwa in the Infinity Castle.
At the heart of the ryokan’s lobby lies a floating stage where musicians in traditional kimono perform live shamisen music from 4:00 to 6:00 PM, welcoming guests with a touch of old-world Japan.
Just as the Infinity Castle is portrayed with vast, multi-level spaces and staircases, Ookawaso mirrors this style. Guestrooms line the valley-facing side of the building, offering panoramic views of the Ookawa River gorge.
The inn’s signature feature is its terraced hot spring baths, connected by wooden stairways and large communal halls designed in a nostalgic style.
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Rooms and Facilities
Ookawaso offers 110 guest rooms, designed in either traditional Japanese or modern Western style:
- Japanese rooms: Tatami flooring with futon bedding, perfect for those seeking an authentic stay.
- Western rooms: Hardwood floors with twin beds, equipped with modern amenities.
Room sizes range from 20 m² to 33 m², providing plenty of space for relaxation.
Unique Hot Spring Baths
The ryokan features two remarkable baths:
- Shiki Butai Tanada – an outdoor bath designed in terraced layers like rice paddies (tanada in Japanese).
- Kuchu Roten Buro – an open-air bath overlooking the breathtaking Ookawa River Valley.
The natural hot spring water here is rich in sulfate minerals, known to ease neuralgia, muscle pain, joint stiffness, and fatigue. Guests can also enjoy separate men’s and women’s saunas—one with Himalayan pink salt and the other with Maifan stone—plus the Miroku Spa, offering restorative wellness treatments.
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Dining and Cultural Experiences
The dining experience at Ookawaso celebrates Kaiseki cuisine, crafted with fresh, seasonal ingredients:
- Kawamonochu Restaurant – serving live cooking performances right in the lobby.
- Kawadoko – a private dining area tailored for intimate meals.
Local specialties include Aizu chicken meatball hotpot, Aizu-style egg rice, and locally brewed sake.
Beyond dining, guests can relax in a tea lounge, browse handcrafted souvenirs at the Aizu gift shop, or immerse themselves in samurai culture at the ryokan’s dojo. Here, visitors can even train in swordsmanship—and if fortunate, meet Tetsuro Shimaguchi, the sword-fighting choreographer for Kill Bill, who occasionally teaches classes.
Location Information
Address: 984 Shimodaira, Ashinomaki, Oto-machi, Aizuwakamatsu City, Fukushima Prefecture
Ashinomaki Onsen Ookawaso is more than just a luxury ryokan—it is a journey into both Japanese tradition and anime fantasy. For Demon Slayer fans and hot spring lovers alike, it offers the chance to step into a real-life Infinity Castle while surrounded by the tranquil beauty of Fukushima’s valleys.
