Koivuniemi was born in Urjala in 1940 and had no architectural training, nor any formal artistic training either. Finnish has a word for what he was — kansantaiteilija, a folk or self-taught artist — and it is applied to him without a trace of condescension. He worked in wood, carving sculptures. Then he worked in stone. Hiidenlinna went up at Hiidenharjuntie 61, in the village of Keltiäinen in the Somerniemi district of Somero: three storeys, 285 square metres of floor, natural fieldstone and concrete, finished in 1992. Somero named him its Mekkopitäjäläinen that same year, and in 1999 voted him the somerolainen of the millennium. The building was never a folly for its own sake. He needed a workshop, and somewhere to hang what came out of it.

So the castle became a gallery. Koivuniemi showed his own carvings there and brought in changing exhibitions, and people came. By 2006 Hiidenlinna ranked as the eleventh most-visited tourist destination in all of Varsinais-Suomi — a region that also contains Turku, the archipelago, and Naantali. That placing is a fair measure of what one determined person can do to a map. There is no marketing department behind an eleventh place like that. There is a man in a stone castle on a lake, open through the summer, who kept the exhibitions interesting enough that word travelled. The setting does some of the work: water on one side, forest on the others, and a structure that carries itself as though it were several centuries older than it is.

By 2008 Koivuniemi had reached retirement age and began searching for someone to take the place on. Not simply a buyer — a buyer who would keep the exhibition activity going. That search took five years. In 2013 Hiidenlinna was sold to Sami Uotila, a skier who ran a ski resort business in Vihti, and who said publicly that he intended to continue roughly along the existing lines. Handing over a life's work with conditions attached is a fragile sort of arrangement, and most such conditions do not survive contact with the next owner's ideas. This one did better than most, though not in the way anyone would have guessed.

From 2015 the site turned into a summer amusement park, and it has been expanding ever since. Around five hectares now hold an adventure park, zip lines, a pumptrack, skate and scooter ramps, a bike park, buggy karting, gold panning, a troll forest, an animal park with reindeer, a Lapland area with Santa's workshop, several restaurants and rental villas. There is a K.I.T.T. car from Knight Rider. It is a considerable distance from a one-man gallery of wood sculpture, and the purist's objection writes itself. But Koivuniemi's castle is still standing in the middle of it and still open to visitors — the original object, with everything else accumulated around it like a reef around a rock.

Somerniemi stopped being its own municipality in 1977, when it merged into Somero. Somero itself became a town on the first day of 1993, though it had been known as a trading place since the fourteenth century. The whole municipality holds 8,339 people across nearly 700 square kilometres, and that density is less abstract once you drive it. Its other great attraction is the opposite of a castle in almost every way: the Häntälä hollows, a landscape of ravines cut by the Rekijoki, grazed for centuries into some of the richest traditional meadow biotopes in Finland and now protected under Natura 2000. Somero sits inside the triangle formed by Helsinki, Turku and Tampere — close enough to all three that a day trip works, far enough from each that it never became anybody's suburb.