The Kuressaare Episcopal Castle is the centrepiece of the town and one of the most remarkably complete medieval fortresses in the entire Baltic region. Built for the bishops of Ösel-Wiek beginning in the late 13th century, the main tower was completed in the 14th century and the outer curtain wall added over the following decades. Unlike many Baltic castles reduced to ruin or rebuilt beyond recognition, Kuressaare's has survived in exceptional condition — you can walk the full circuit of the ramparts, explore the vaulted halls and cellars, and emerge on the towers with a clear view across the bay.
The Saaremaa Museum occupies the castle interior and covers the island's history from prehistory to the Soviet period. Exhibits include Bronze Age finds, medieval weaponry, and documents from the island's complex past as a Danish, Swedish, Russian, and finally Estonian territory. The castle is surrounded by a moat and a public park that is pleasant to walk even without entering the castle itself. Open daily in summer, reduced hours in winter; admission around €7.