Built under Harald Bluetooth between 958 and 987 AD, Borgring is a UNESCO World Heritage Site and one of the best reasons to have a few extra hours in Køge. The fortress is 145 metres across, making it the third-largest of Denmark's ring fortresses. The earthwork ramparts are still clearly visible, and the site has been set up for visitors with interpretation boards and walking paths.
An experience centre opened in 2025, giving context to the archaeology and to Harald Bluetooth's wider project of consolidating power across Denmark. The site is about 4 km from central Køge — reachable by bike or taxi, or on foot if you don't mind a pleasant country walk. It is worth going even without the centre; standing inside the ramparts and trying to picture the garrison barracked here in the 10th century is its own reward.