Kastellet, Copenhagen

Start with the ground under your feet. The path rises gently, and suddenly you are walking on top of a wall made of earth and grass. To one side, still water lies in a moat. To the other, rows of long red buildings stand in neat lines below you. You can hear joggers' footsteps, the call of a gull, perhaps a bugle from somewhere inside. This is Kastellet, and it is one of the best-preserved star fortresses in Northern Europe.

From the air, the shape is clear. It is a five-pointed star, ringed by water. Each point is a bastion, an angled platform that juts outward. From the ground, the star is harder to read. You follow the ramparts and feel the path turn sharply at each corner. That shape was not decoration. It was a calculation, designed so that defenders could fire along the walls and leave attackers no blind spot.

The story begins in the early seventeenth century. King Christian the Fourth, the great builder-king of Denmark, ordered a fortification here in the 1620s. It guarded the northern approach to the city and the harbour entrance. The first version was smaller, an earthwork known as Saint Anne's Redoubt. It was a modest beginning for what would become the anchor of Copenhagen's defences.

Then came war with Sweden. In the late 1650s, Swedish forces besieged Copenhagen, and the city very nearly fell. The lesson was hard and clear. After the war, King Frederik the Third rebuilt and enlarged the fortress into the star you see today. A Dutch military engineer, Henrik Ruse, is generally credited with the design. The Dutch were the masters of this kind of earth-and-water defence in that era.

Why earth, and not tall stone walls? By the seventeenth century, cannon could smash high masonry. Thick banks of packed earth simply absorbed the shot. Low, broad, and grassy, these ramparts were built to swallow impact. So the soft green slopes that look so peaceful today were, in fact, the most modern military technology of their time. Here is the first surprise: the gentle landscape is the weapon.

Now look down into the interior. Those long red buildings with steep roofs are the old barracks, laid out in tidy rows. Soldiers and their families once lived in them, inside a small, self-contained town. There were stores, workshops, a church, and a commander's residence. Life inside the walls followed the rhythm of drills, bells, and the opening and closing of the gates.

And here is the second surprise. Kastellet is not a museum. It is still an active military site, used by the Danish defence forces. Uniformed personnel still work in some of these buildings. That is why certain doors stay closed and some areas are off limits. You are walking through a working base that the public is also allowed to enjoy. Few fortresses of this age still serve their original masters.

At the edge of one rampart stands a windmill, dark and wooden against the sky. A windmill seems out of place in a fortress, but it made perfect sense. A garrison under siege needed to grind its own grain. A mill on the walls meant bread without opening the gates. The mill you see dates from the nineteenth century, but it continues a very old idea: a fortress must be able to feed itself.

The fortress church sits inside the walls, a simple building from the early eighteenth century. Its quiet exterior hides a darker detail. Prison cells were built against its walls. According to accounts of the fortress, prisoners could follow the church services through openings into the nave, while remaining unseen by the congregation. Kastellet served as a prison for centuries, and some notable political prisoners were held here.

The fortress also carries a heavier memory from the twentieth century. On the ninth of April, 1940, German forces landed nearby by ship and occupied Kastellet in the opening hours of the invasion of Denmark. The occupation of the country had begun. After the war, the fortress returned to Danish hands. Today, a memorial near the grounds honours Danes who lost their lives in international service.

There are two main gates. The King's Gate faces the city to the south, and the Norway Gate faces north, a reminder of the centuries when Denmark and Norway were united under one crown. Each gate is a passage through the thick earthworks, with a bridge across the moat. Walk through one slowly. You will feel the change in sound as the city noise fades behind the walls.

Kastellet also sits at the heart of a well-loved walk. To the east lies the harbour promenade, where the small bronze Little Mermaid sits on her rock. To the south, you can reach the Gefion Fountain and the English church of Saint Alban. Many visitors loop the ramparts, then continue along the water. For Copenhageners, this is a place for morning runs, Sunday strolls, and quiet benches.

Pause for a moment wherever you are standing now. Notice how calm it is, how the water holds the reflection of the trees, how the red walls glow in the low northern light. This star was built for war, for siege and cannon. Today it holds birdsong, dog walkers, and the steady crunch of gravel underfoot. As you walk away, look back once at the sharp corner of a bastion, and remember what that angle was made to do.