I am sitting on a wooden bench, watching nursery school kids hold a rope and walk in a single file on an afternoon outing. Cheeks flushed from the heat, they rest near a pond with rainbow-coloured koi gliding past them. Some of them lean over the pond, fascinated by the movements, while their teachers wash their hands, scooping water from bamboo ladles in the spring water; others throw bread pieces to the always-hungry carp which gobble them ferociously. 
I am in Shimabara, a relaxed and laidback castle town in the Nagasaki prefecture of Kyushu in Japan, known as the 'City of Springs' with more than 60 springs. The town sits on a peninsula in the shadow of the active volcano, Mount Unzen. The first springs of the city appeared after the eruption of the mountain in 1792. The waters of Shimabara have been voted as one of the '100 remarkable waters of Japan' by the Ministry of Environment. "The underground water of the town is the purest, thanks to the volcanic filtration process - filtered through volcanic rock from the volcanic Mt Unzen for years," explains our guide Toshi san. The water is enriched by various minerals including magnesium, calcium, and silica. Even the signature dish of the town, Kanzarashi - special rice flour dumplings - is cooled with the special spring water and then dunked in syrup!
Driving down to Shimabara from Nagasaki, I have driven through miles and miles of roads flanked by terraced neon rice paddy fields, red cows, meandering canals and mountains. Beneath the peaceful landscape, Shimabara hides a turbulent past when, in 1618, it was the place where the Shimabara Rebellion happened. Repressed farmers, merchants and fishermen, who had converted to Christianity, revolted against the policy of the Matsukara Clan that imposed high taxes to finance the building of their extravagant castle. The revolt was led by a young boy named Amakusa Shiro. 
Catholicism came to Shimabara in 1563 and the ruler, Arima Yoshisada, gave permission to Jesuit missionaries to build churches and seminaries. But later, as Japan was unified, they feared an invasion by the Christians and banned Christianity. Christians were killed and tortured. The Tokugawa Shogunate sent an army and, after a lengthy siege, defeated the rebels. But Matsukura was punished for his mismanagement and was the only leader to be beheaded during the Edo period.
I visit the white-walled Shimabara castle first built in 1624 and later reconstructed in the 1960s, where five tiers of the turrets have exhibits from the past: from Christian statues revered by hidden Christians, to samurai armours, to swords and weapons. The moats and the walls are the only parts of the castle that are original and still stand tall. From the terrace of the castle, we get a panoramic view of the town, the Ariake Sea and the surrounding mountains. 
To the west of the castle grounds are the old Samurai homes or the Buke Yashiki, where lower order soldiers subsisted on meager salaries. This was called Teppo Machi or Gun Town. We walk along the long narrow street with houses behind volcanic stone walls, and a central canal of spring water, dotted by ferns..  I can almost imagine a katana-wielding samurai walk down the atmospheric street in yesteryears. This is where, once a year, the Water Festival is celebrated with bamboo lanterns lighting up either side of the canal. Three samurai homes from the Meiji era, with thick, thatched roofs and eaves, are open to the public with minimalistic Zen interiors of tatami mat floors, sliding doors and alcoves. Some have mannequins of the samurai and their families and re-created kitchens. The samurai sit at low desks writing letters, others pose in the kitchen, cooking. I realise that the samurai were not just warriors - they also functioned as civil servants, clerks, and accountants. 
If you are a foodie then don't leave town without trying the local speciality called Guzoni. Legend has it that this dish originated during the Shimabara Rebellion when Amakusa and his rebels were holed up in the castle and all they had was mochi and vegetables. This hearty peasant soup made of mochi-style rice balls, meat, fish and vegetables is on every local menu today.
Walking through the town, I come across spring water drinking corners and the Yutorogi footbath, a free footbath with the hot spring water piped in, where locals relax on wooden benches with newspapers or catch up on gossip with neighbours. I arrive at the 'street of swimming carp' in the Shinmachi area. Here, the limpid waterways that line the streets are rife with more than 1,000 colourful koi (carp) fish that were introduced by the local neighborhood association to beautify the streets and attract tourists.  
The area is dotted with picturesque wooden teahouses set amid manicured gardens and babbling creeks and koi ponds that are open to the public. I head to the Yusui Teien Shimeiso - a tea home on stilts over the spring water - where I sit dangling my feet over the water, sipping on green tea and soaking in the tranquility of this zen space. I am mesmerised by the sight of the koi swimming and the beautiful cypress and pine trees that frame the scenery. There is a Japanese phrase "koi no taki-nobori" - that translates into "Koi climbing rapids". "The carp are revered in Japanese culture as they are a symbol of luck and prosperity and overcoming obstacles and triumph over adversity - as they swim upriver," explains Toshi San.