The Round Tower or Rundetarn is the oldest observatory in Europe and was used by the astronomers of Copenhagen University through the 1800s. Its most unique feature is the spiral cobblestone ramp that winds around its center, made large enough that a horse and carriage could haul heavy astronomical equipment to the top. Halfway up is a museum and exhibition space, once the university library, and the city occasionally hosts astronomy viewings at the top of the tower. Admission is 25 DKK; children under 5 free.