Much of the Old Town, the Stare Miasto, was destroyed during World War II. Though it was rebuilt in a traditional style, it retains a clean, cheerful 1960s-to-'70s feel, complete with surreal Socialist-era motifs on what were rebuilt to look like baroque and Renaissance buildings. The streets toward the Lyna River are particularly nice and atmospheric. Most of the sites in town center on the 14th-century castle, Zamkowa 2 (tel. 089/527-95-96; www.muzeum.olsztyn.pl), originally constructed by the Teutonic Knights but thoroughly rebuilt since, tucked in behind the Old Town. You can climb to the top of the tower for a commanding view of the town and surroundings. A small museum (June-Sept Tues-Sun 9am-5pm; Oct-May Tues-Sun 10am-4pm) can fill you in on the region's Polish history, and there's a small room in the castle where the astronomer and jack-of-all-trades Nicholas Copernicus once lived. He once apparently commanded a Polish garrison here while under siege by the Teutonic Knights.
