Once upon a time, in a country far, far away - about 500 years ago, in the small Dutch market town of 's-Hertogenbosch, to be precise - there lived a painter who had an outlandish imagination.

Like other European artists of his time, he mainly painted scenes from the Bible, the lives of the Christian saints, and heaven and hell. But his pictures were unlike anything seen before. Every corner of them teemed with half-human monsters, giant birds, enormous fruit and other bizarre images. The devil was in the details, and all around his holy subjects the details were often devils.

The painter's skill and creativity in showing both the natural and the supernatural worlds attracted commissions from European royalty, and although he was but a humble craftsman, this led to his elevation in society. He married a woman from a well-to-do family and began to sign his paintings with a name that told buyers where to find his studio: Jheronimus Bosch.

This year his picturesque hometown, also known as Den Bosch, an hour's train ride south of Amsterdam, is celebrating the 500th anniversary of the death of its most famous son, widely known today as Hieronymus Bosch. His house still stands on the north side of the triangular marketplace, one of more than 500 mostly medieval listed buildings lining a labyrinth of narrow streets and the network of canals that lace what is one of the Netherlands' prettiest and best-preserved historic cities.

Paintings by Bosch's contemporaries on display at the Noordbrabants Museum show how little the older parts of the city have changed. Some of it appears in the background of Bosch's own works, as do the skylines of other medieval towns still identifiable in the distance from atop the city's substantial walls.

Despite possessing no Bosch paintings or drawings itself, the Noordbrabants has managed to persuade Madrid's Prado, Paris' Louvre, Venice's Accademia and dozens of other prestigious museums to allow their Bosch masterpieces to make a temporary return to the town where they were created, forming the greatest exhibition of these visionary works ever assembled.

Tickets for the exhibition, which closes next weekend, sold out long ago, but for the year-long celebrations, Bosch's monsters have stepped out of their canvases and into 's-Hertogenbosch's streets. They appear in magnified and gaudy 3D against the backdrop of the town's sober three-storey facades of dark brick with stepped gables sometimes surreally lozenged by subsidence.

There are surprises round every corner. A giant pig in a nun's wimple stares across a canal. A robot-armed creature with the skull of a bird and playing a harp peers from behind a tree. A red-booted blue beast atop a canal-side mooring post plays a flute which is actually its own head.

These seemingly very modern images, apparently anticipating surrealism by several centuries, are combinations of symbols that would have been easily readable by Bosch's medieval audience. Skating creatures indicated unreliability or folly, owls implied not wisdom but evil and giant musical instruments suggested eroticism. Despite his influence on everything from Salvador Dali to Star Wars, Bosch perhaps simply intended to give clear warnings about the dangers of a life given over to pleasure.

The Noordbrabants Museum may temporarily house much of Bosch's oeuvre, but the Jheronimus Bosch Art Center, housed in St Jacob's church, shows his entire body of work year-round in reproduction. Even the ancient frames have been recreated, and triptychs long ago sawn up and distributed between various museums can be seen reassembled. Visitors are encouraged to swing them open and discover their interiors for themselves. The crypt now houses a tableau of Bosch at work in his studio, showing the drying of the wood panels on which he painted and their preparation with the white compound known as gesso, on which Bosch sketched a painting before adding colour. The museum's tower gives fine views of the city's roofscape and across to the soaring Gothic facades of its greatest late-medieval building, St John's Cathedral.

The cathedral was still under construction in Bosch's day, and he knew the architect. As a boy he would have seen what usually remains hidden - the dozens of small statues later hoisted to sit astride the slender flying buttresses that support the cathedral's upper storeys, remaining largely out of sight from ground level. For this year only, there's a once-in-several-centuries chance to climb up and view them close up.

It seems the masons were given a free hand and there's a surprising absence of religious themes, although a pelican with its long beak deep in an inkpot is the official symbol of the church's patron saint. Burly figures who may be likenesses of the builders themselves tip back large flagons of ale, or clutch at their stomachs and give great belly laughs, while clowns dance and musicians thrash at their instruments. Never did solid stone seem so full of rude life. There are lions and eagles, but also two-legged half-animal monsters that seem lifted straight from Bosch's art, although the claim is that perhaps the influence went the other way. One figure cradles an owl, a subject popular with Bosch, and one piper and a sole female figure with a memorably complicated hairstyle are said to be individually identifiable in his works.

Such playfulness is part of the local Brabant Gothic style, which includes the recent addition of an anachronistic angel in jeans using a mobile phone.

There's a climb of 180 stairs, but this rare chance to stroll along sturdy wooden rooftop walkways pursued by the cathedral's tuneful bells to view one possible source of Bosch's inspiration would in itself make a trip to Europe worthwhile. But then even without the art, 's-Hertogenbosch deserves a visit. Getting lost in its mini-maze of historic buildings or sitting in one of innumerable cafés consuming a large chocolate-covered cream-filled profiterole, called a Bossche bol, are pleasures enough.

And however vivid, Bosch's warnings against a life given over to pleasure can be quietly ignored.

For details of exhibitions, live events, the "Wondrous Climb" to the roof of St John's Cathedral (available until October 30), canal boat trips and a son-et-lumière, go to www.bosch500.nl.