Vestvolden is the long western rampart of Copenhagen's 19th-century land defences, running for more than 14 km through today's suburbs. Parts are restored, but the best stretches are the quieter overgrown ones, where old barracks, magazines, moats, and earthworks appear between trees and bike paths.
It is a good reminder that Copenhagen once worried about attack from land as much as from the sea. Start near an S-train station and walk or cycle a section rather than trying to see the whole line in one go.