A well-rubbed bronze of Copernicus welcomes you to Olsztyn's massive red-brick 14th-century castle, the most important historic building in town. And it's the Copernicus association that makes this the town's top sight – the astronomer as administrator of Warmia lived in the castle from 1516 to 1520. He made some of his astronomical observations here, and you can still see the diagram he drew on the cloister wall to record the equinox and thereby calculate the exact length of the year.
The rest of the collection is an eclectic mix of rural knickknacks, dubious art and various temporary exhibitions. The tower is hardly worth the climb, though there is some interesting graffiti on the inside of the viewing platform left by visitors from down the decades.