Copenhagen's seventeenth-century city ramparts came down in the nineteenth, and where the walls stood, the city planted a green ring. The walk traces the ghost of the old fortifications: Tivoli (gardens built on the former ramparts in 1843), Botanisk Have, Ørstedsparken, Østre Anlæg, and Kastellet — the star fort that was never demolished and is still working.
Five kilometres, two hours at a strolling pace. Start at Tivoli's main gate at Rådhuspladsen — pay to go in if you've never, otherwise the perimeter is the walk's first stretch. North past the Botanical Garden's Palm House to Ørstedsparken with the lake and the H.C. Ørsted bust. Cross Nørre Voldgade into the cultural cluster — Statens Museum for Kunst and Hirschsprung are book-ended by Østre Anlæg's pond and lawns.
The walk peaks at Kastellet, the 17th-c star fortress still in military use, the earthen ramparts walkable in a slow loop. Den Lille Havfrue sits a few hundred metres beyond on her rock at Langelinie — overphotographed and smaller than you expect, but the closing of the ring is here. Østerport S-tog is fifteen minutes south for the train back.