COPENHAGEN. View from Round Tower (Rundetaarn): First built as an observatory by King Christian IV in the 1600s — one of the oldest in the world — the Round Tower still serves as an observatory and now boasts a library and exhibition space. Visitors can make their way skyward by way of an internal spiral walkway. Open daily 10 a.m. to 5 p.m., until 8 p.m. in summer months. From mid–October to mid-March on Tuesday and Wednesday, both the tower and the observatory are open 7-10 p.