When construction began between 1198 and 1209, Deventer was at the height of its Hanseatic prosperity. The harbour ran almost up to the church's footings. Ships from Lubeck, Bergen and the Baltic tied up within sight of the masons, and so it made sense to dedicate the new basilica to Saint Nicholas, patron of sailors. The architecture told the same story: the heavy Romanesque mass, the round-arched aisles, the brick coursing all share family resemblances with churches around the Baltic Sea, because the same builders, the same fashions and the same trading routes shaped them all. To pray here was to admit, quietly, that your fortune was in salt water.

The legend goes like this. Two sisters from Deventer, Martha and Beatrix, fell for the same visiting knight. Beatrix married him. Martha, left behind, used her inheritance to build a church on the mountain - the modest sandy rise the town centre sits on - and ordered two towers, one slightly taller than the other, because she and her sister had been different heights. "Two children of one father alike and inseparable," the story has Martha say. "So that there will eternally remain the memory of our love." The west tower, on your right as you face the church, is the larger one. Whether or not a Martha ever lived, generations of Deventer children have been told this story to explain a building that does, in fact, sit there with two slightly mismatched spires, looking like a memorial to something a little sadder than faith.

There is a second, harder story embedded in the church too - literally. Stuck in the exterior masonry of the west tower is a large round stone, the size of a fist, said to be a cannonball lodged there during a seventeenth-century siege. The version told in Deventer involves river pirates working the IJssel. The Netherlands had pirates aplenty during the long Dutch wars with Spain and the maritime free-for-all that followed, and the IJssel was a busy commercial artery worth raiding. Whether the stone is genuine ordnance or just a stone the locals decided ought to have a story, it has been doing its job for centuries: making people stop, point and ask.

In 1580 the Dutch Reformed Church took over and stripped the building down to Calvinist plainness. Catholic statuary went out. Murals were whitewashed under chalk. The basilica was renamed the Bergkerk - the Mountain Church - and used as a Protestant parish for nearly four more centuries. Then in 1967 it was disestablished entirely. The municipality took the keys, and the building began its third life as an exhibition hall and concert space. From 1991 to 2005 it housed temporary shows for Museum de Fundatie, and today its long Romanesque nave hosts art installations, classical recitals and the occasional festival. Inside, the columns are old enough to have heard Latin masses, sober Reformed sermons and now amplified cello. The acoustics, conveniently, are excellent for all three.

Climb to the upper galleries on an open day and the view explains the whole town. The IJssel curves west, broad and slow. Red-tiled roofs run down to the river in a tight medieval grid. The Brink, Deventer's old market square, is just a few minutes' walk away. From up here the Bergkerk's older identity - merchant church, navigational landmark, declaration of a town's commercial ambition - feels more legible than its later ones. The towers were built to be seen from the river, by sailors looking for home.