Dybowski Castle Ruins stand across the Vistula from Torun's old town. The castle was built by King Wladyslaw Jagiello in the 15th century and later fell out of use, leaving a rough shell of walls.
You can explore the ruins informally, though the masonry is uneven and some sections need care. The river setting makes it a good add-on to Torun's more polished medieval sights.