名胡桃城 · Nagurumi-jo
The tiny castle whose seizure triggered Hideyoshi's Odawara campaign — Japan's unification started here on a narrow Gunma ridgeline in 1589.
#115 — Continued 100 CastlesBus also available
Free admission. The ruins are preserved as a public park and National Historic Site. No structures survive — earthworks and stone remnants only.
Nagurumi Castle is one of those rare sites where the historical significance is completely out of proportion to the physical remains. The earthworks are well-preserved and the horikiri ditches are a textbook example of mountain castle defense design. But the real reason to visit is the history — stand on this modest ridgeline and understand that the act of seizing this small outpost set off the chain of events that unified Japan. Combine with Numata Castle (15 minutes by car) for a full Sanada clan day.
Nagurumi Castle is historically tiny — a minor Sanada clan outpost on a ridge above the Tone River. But its seizure in 1589 by a Hojo retainer triggered Toyotomi Hideyoshi's massive Odawara campaign, which ended with the fall of the Hojo, the unification of all Japan under Hideyoshi, and the establishment of the political order that led directly to the Tokugawa shogunate. No other castle in Japan had such outsized consequences for such modest size.
Nagurumi Castle was built by the Sanada clan to control the Tone River valley between Numata and the mountains to the north. Its position on a narrow ridge made it a classic yamajiro — compact, difficult to approach, and commanding the river route below. The Sanada used it as part of their network of mountain strongholds that allowed them to maintain independence between the great powers of the Sengoku period.
Despite its tiny size and the absence of any standing structures, Nagurumi Castle's earthworks are among the best-preserved examples of a compact Sengoku ridgeline fort in Kanto. The series of defensive ditches (horikiri) cutting across the ridge, the small main compound, and the connecting approach earthworks are all clearly visible and well-maintained as a National Historic Site.
Visitor Tip
Nagurumi Castle is a great introduction to yamajiro (mountain castle) earthwork design. The horikiri ditches are clearly visible and easy to understand as defensive features. The ridgeline walk from the parking area to the Honmaru takes about 10-15 minutes and is not strenuous. The historical significance is wildly disproportionate to the castle's physical size — read the interpretive signs carefully.
yamajiro
Mountain castle — compact ridgeline fort above the Tone River valley, relying entirely on terrain for defense
nawa
Rope-style — compounds arranged in linear sequence along the narrow ridge, connected by a single approach path
Ruins — earthworks, defensive ditches (horikiri), and stone remnants survive; no standing structures
nozurazumi — Natural stone remnants — minimal stonework given the small scale of the castle; earthworks dominate
Nagurumi Castle relied primarily on earthwork defenses — ditches cut across the ridge (horikiri) and raised earthen platforms (kuruwa) carved from the ridgeline itself. Stone walls were minimal at this small frontier outpost.
Horikiri (Ridge-Cutting Ditches)
Ditches cut perpendicular across the narrow ridge prevented attackers from moving along the ridgeline approach. Anyone assaulting the castle had to descend into and climb out of each successive ditch under fire from the compound above — exhausting and deadly.
Ridge Constriction
The natural ridge narrows to the point where only a few men can advance abreast, making frontal assault impossible on any scale. The defenders' advantage in these conditions was overwhelming — a small garrison could hold a much larger attacking force indefinitely.
River Gorge Below
The Tone River gorge below the castle's flanks made flanking approaches impossible. The only viable approach was directly along the ridge — funneled directly into the castle's prepared defenses.
Death from the Side
Attackers enter the corridor between walls. The path seems straightforward — but it isn't.
Historical Context — Nagurumi Castle
Attacking Nagurumi Castle meant advancing single-file along a narrow ridge while under fire from the compound above, then repeatedly descending into and climbing out of horikiri ditches. Each ditch forced a complete halt under concentrated defensive fire. The river gorge made flanking impossible. A garrison of even 50-100 men could theoretically hold this position against a force many times its size — the problem was supply and relief, not direct assault.
Sanada Masayuki builds Nagurumi Castle as part of the Sanada clan's mountain fortress network in the Joshu highlands. The castle guards the Tone River valley approach to Numata Castle and serves as an outpost in the contested border region between Sanada, Hojo, and Uesugi territories.
A Hojo retainer seizes Nagurumi Castle in violation of the peace agreement that Toyotomi Hideyoshi had brokered between the Sanada and Hojo clans. Hideyoshi, who had issued an order prohibiting all unauthorized military action, uses this violation as his legal pretext to declare war on the Hojo.
Hideyoshi launches the Odawara campaign — one of the largest military operations in Japanese history, with over 200,000 soldiers converging on the Hojo stronghold of Odawara Castle. The Hojo surrender after a siege of approximately three months. The fall of Odawara effectively completes the unification of Japan under Hideyoshi.
Following the Hojo defeat, Nagurumi Castle is rendered obsolete by the post-unification peace and is subsequently abandoned.
Nagurumi Castle is designated a continued 100 Famous Castles site, bringing renewed attention to this historically pivotal but physically modest ridgeline ruin.
The NHK Taiga Drama about Toyotomi Hideyoshi depicted the Nagurumi Castle incident as the trigger for the Odawara campaign — the pivotal moment that allowed Hideyoshi to unite Japan.
Nagurumi Castle's role as the trigger for Japan's unification makes it a recurring reference in Sengoku period historical fiction and strategy games.
Spring and autumn for best walking conditions. Avoid winter as the mountain paths can be icy.
45 minutes to 1 hour
Walk the full ridgeline to the Honmaru rather than stopping at the first compound — the view from the summit over the Tone River valley is the payoff, and the series of horikiri ditches you cross on the way up make the castle's defensive logic immediately clear.
Free Entry
Free admission. The ruins are preserved as a public park and National Historic Site. No structures survive — earthworks and stone remnants only.
Open at all times. Winter snow may make the mountain paths slippery.
沼田城 · Numata-jo
📍 Gunma — Kanto
Sanada clan cliff fortress above three river gorges — one of Sengoku Japan's most dramatic natural defensive positions, destroyed by Tokugawa political fiat in 1681.
岩櫃城 · Iwabitsu-jo
📍 Gunma — Kanto
The Sanada clan's ultimate mountain refuge — one of Sengoku Japan's most dramatically positioned castles, now famous for sea-of-clouds autumn photography.
箕輪城 · Minowa-jo
📍 Gunma — Kanto
The castle that resisted Takeda Shingen and Uesugi Kenshin — a vast earthwork system in Gunma preserving the memory of the Nagano clan's remarkable defense.
The nearest station is Jomo-Kogen Station (JR Joetsu Shinkansen). It is approximately a 40-minute walk from the station. Local bus toward Minakami from Jomo-Kogen Station. Taxi available from Jomo-Kogen Station. Parking: Small free parking area at the castle ruins site. Accessible with a JR Pass.
Nagurumi Castle is free to enter. Free admission. The ruins are preserved as a public park and National Historic Site. No structures survive — earthworks and stone remnants only.
Nagurumi Castle is one of those rare sites where the historical significance is completely out of proportion to the physical remains. The earthworks are well-preserved and the horikiri ditches are a textbook example of mountain castle defense design. But the real reason to visit is the history — stand on this modest ridgeline and understand that the act of seizing this small outpost set off the chain of events that unified Japan. Combine with Numata Castle (15 minutes by car) for a full Sanada clan day.
Nagurumi Castle is open 00:00 – 23:59 . Open at all times. Winter snow may make the mountain paths slippery.
Plan on spending 45 minutes to 1 hour at Nagurumi Castle. Walk the full ridgeline to the Honmaru rather than stopping at the first compound — the view from the summit over the Tone River valley is the payoff, and the series of horikiri ditches you cross on the way up make the castle's defensive logic immediately clear.