Now, with the Japanese government lifting some restrictions on business travellers on November 8, there is growing anticipation that the rules covering tourists will also soon be relaxed.

That cannot come soon enough for Jinkawa and the operators of Odawara Castle Park, which opened its Ninja Museum in the grounds of the 15th-century fortress in April 2019 to explain the history of the legendary warriors, but has had to rely on domestic visitors since the pandemic broke out.

Jinkawa says a 90-minute online experience offers a fascinating insight into ninja, but to be within the walls of the imposing castle adds an extra realistic element.

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Jinkawa’s training sessions for wannabe ninja starts with the most fundamental skill that must be mastered: breathing.

Within a recreation of a traditional feudal-era wooden house inside the museum – which holds a few secrets for eagle-eyed visitors – he makes us sit cross-legged but very upright. We are told to close our eyes, think of nothing and “centre” ourselves. We must concentrate on breathing, four seconds inhaling and six seconds exhaling, to prepare for the warrior training that is to come.

The next attribute that a ninja must have is a loose, relaxed stance, with legs apart – but ready to spring into action. Sword work is the next drill, and clearly one of the most important skills of the spies and mercenaries who can trace their shadowy history as far back as the 12th century.

Jinkawa demonstrates the two-handed grip that is required on the handle of the weapon – for us novices, blunt wooden versions, thankfully – and he puts us through our paces, first with cuts from above the head that swing down in a violent but controlled arc, then with slashes from side to side as the warrior advances or backtracks on the balls of his nimble feet.

Disappointed in my performance, he suggests that it would take far more than the 1,000 drills a day expected of a novice ninja to get me into shape.

We move on to another weapon closely associated with ninja, although Jinkawa says the connection is inaccurate. Although shuriken throwing stars are the staple of legend and ninja movies, the warriors themselves usually wore them as amulets, to ward off evil, and would only throw them as a last resort.

Instead, they would turn whatever quickly came to hand into a throwing weapon and, fortunately, Jinkawa has brought some tapering chopsticks with him.

My initial scepticism of being able to ward off a rampaging enemy with an eating implement is quickly dispelled as he shows me how to hold it between the first two fingers of either hand, releasing it just as my arm comes down from the vertical to the horizontal.

It is surprising how much velocity the improvised weapon picks up and I leave a plastic board liberally peppered with holes. Get one of those in your enemy’s eyes and it will sting.

The interactive museum has a theatre that provides an overview of Odawara’s Fuma clan of ninja and places where children can practice climbing walls and ropes to infiltrate an enemy’s fortress. Instructors teach visitors skills such as concealment, disguise and how to silently creep up on a foe.

Back in the grounds of the castle, I feel ready to take on all-comers.

Odawara Castle Park sprawls over more than 106 hectares (260 acres) in the heart of this port town, which was strategically important in the feudal era as it controlled the narrow plain between the ocean and the mountains that rise steeply a short way inland. All coastal roads between the ancient capital in Kyoto and the emerging power centre of Edo, modern-day Tokyo, had to pass within a few miles of the castle.

The Hojo family used their power to expand their influence over a large part of what is today the Kanto region. Instrumental in that were the Fuma ninja, who were by day ordinary members of the community but could be called upon in times of crisis to serve their lords.

History does not record precisely who the leader of the ninja was, but his name was thought to be Kotaro Fuma and the only surviving description of him says that he was tall and powerfully built.

Despite having a cohort of ninja at their disposal, the Hojo’s rule came to a sudden end in 1590 with the siege of Odawara, at the hands of the warlord Toyotomi Hideyoshi and his vast army. The surrounding areas and main keep of the castle were expanded until it reached its zenith in the early 1630s, only to be destroyed by a massive earthquake in 1633.

The site was eventually sold to Odawara city and, in the last couple of decades, work to return the citadel to its former glory has been completed.

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A series of bridges cross the broad moat, lined by cherry trees that are spectacular in the brief blooming season in April, and lead to fortified gates and courtyards that have been rebuilt just as they once appeared, including loopholes for archers and rudimentary rifles of the period.

Atop the low hill at the centre of the complex is the dazzling white castle itself.

Built in a classic Japanese design upon sloped stone foundations, the castle towers more than 27 metres (90 feet) above the courtyard. It is here, on the fifth floor, with stunning views across Sagami Bay to the south and the foothills of Mount Fuji in the opposite direction, that a second unique experience is offered: mindfulness.

Tomomi Iwayama – known as the Flying Monk – works on meditation and mindfulness with major corporations around the world, and also offers sessions online. For our in-person experience, we are invited to sit cross-legged and straight-backed on square cushions on floor and, once again, focus on breathing in and out from the very depths of our bodies.

Emptying my mind does not come easily and I quickly find my thoughts roaming. It is a struggle to bring them back and to focus solely on smoothly inhaling and exhaling.

With daily practice, Iwayama says, it should not take long to get up to 30 minutes of mindfulness. My struggles beyond the 10-minute mark are compounded by crippling pins and needles in my legs.

For anyone taking part in one of Iwayama’s online sessions, it might be easier to cheat. The disadvantage of doing it in person is that he knows when I’m not being completely mindful.

It’s the one raised eyebrow that gives him away.