If you only have time for one of Visby's church ruins, make it Sankt Lars. Built in the 12th century, possibly modelled on Byzantine churches in Constantinople by Hansa merchants who had been there, it has a centrally planned, almost square layout — unusual in medieval Scandinavia. The roof is long gone, but the four-storey thickness of the walls means most of the original structure stands intact, including the great rose window in the apse.
The interior is open and free; in summer it doubles as a concert venue for the Visby Music Week. The acoustics under those open walls are remarkable — voices carry up and out toward the sky.