Considered the oldest of Warsaw’s churches, St John's was built at the beginning of the 15th century on the site of a wooden church, and subsequently remodelled several times. Razed during WWII, it regained its Gothic shape through postwar reconstruction. Look for the red-marble Renaissance tomb of the last dukes of Mazovia in the right-hand aisle, then go downstairs to the crypt to see more tombstones, including that of Nobel prize–winning writer Henryk Sienkiewicz.