Prizren's most important site is the Orthodox Church of Our Lady of Ljeviš, a 14th-century Serbian church that was used as a mosque by the local population until 1911. The church was badly damaged in 2004 by the town's Albanian population and placed on Unesco's World Heritage in Danger list in 2006. Restoration work on its magnificent frescoes has recently begun.
Given its location, the church is not exactly welcoming; it's surrounded by barbed wire and closed except when visitors come to see it. You'll need to present yourself first at St George's Church on the other side of the river to get approval to visit from one of the town's few remaining Serbs. This is well worth doing, however, as even though the frescoes in the church are heavily damaged, there are some stunning, ancient wall paintings here.