The Castle of the Warmian Chapter sits on a bluff above the Lyna river, a hulking red-brick complex begun in the 1340s. The most famous resident was Nicolaus Copernicus, who served as administrator of Warmia in 1516–21 and again later. He lived in the southeast tower, defended the castle from a Teutonic Knights' siege in 1520, and — much earlier in his time here — drew an astronomical table directly onto the cloister wall to track the position of the sun.
Today the castle houses the Museum of Warmia and Masuria, with a permanent exhibition on Copernicus alongside coins, weapons, and ethnographic collections. The cloister with its surviving astronomical drawing is the highlight; allow an hour or two.