Bratislavský hrad
With roots dating back more than a millennium—it was first mentioned in 907 for its role in a battle between Bavarians and Hungarians—Bratislava Castle was significantly rebuilt in the Renaissance style in the mid-16th century. It's this enormous, rectangular form with four stocky towers that you can see today (though it incorporates architectural features from throughout its history, and the dazzling white paint job is distinctly 20th century). Walk up the (steep) castle hill and pass through one of the four entrance gates (probably Viedenská brána or Leopoldova brána) for incredible views of the town and the Danube below. The grounds are free to enter, so you can soak up the vistas as long as you like; make sure you visit the beautiful Baroková záhrada (Baroque Garden) while you're at it. You only need to pay if you want to head inside, either for the SNM-Historical Museum, which is a little sparse but does include access to the Crown Tower via a narrow passageway, or any of the regularly changing temporary exhibits.