The Swedish king, however, was not content with his stunning victory, and at the Swedish Council held at Gottorp on July 7, Charles X Gustav resolved to wipe his inconvenient rival from the map of Europe. Without any warning, in defiance of international treaty, he ordered his troops to attack Denmark-Norway a second time.
The Swedish armies quickly occupied all of Denmark, apart from the capital, Copenhagen, which after a failed assault was swiftly surrounded, in the hope of breaking the defence by starvation. In November 1658 however a Dutch relief fleet under Lieutenant-Admiral Jacob van Wassenaer Obdam defeated the Swedish fleet in the Battle of the Sound and lifted the sea blockade so that supplies and an auxiliary army could reach the capital. The Dutch were afraid a complete Swedish control of the Baltic would ruin their profitable trade in this area.
The Copenhageners had been forewarned by spies, so they had planned their defences well and stockpiled weapons and ammunition.
The walls of Copenhagen bristled with about 300 pieces of cannon, mortars and other artillery, while a diverse mixture of weapons, ranging from muskets and arquebuses to morningstars, scythes, boiling water and tar had been readied for action. Craftsmen, students and other civilian citizens were divided into nine companies, and each of these companies was allocated a part of the wall to defend. The professional soldiers were stationed at the outer field works, the Kastellet and the Slotsholmen.
The Swedish army consisted of about 10.000 professional soldiers, while the Danish defenders, a mixture of professionals (among which a mercenary regiment paid by the Dutch), militia and raw civilians, were of an equal number.
The Swedes started the action by making a diversionary attack at Christianshavn and Slotsholmen at the evening on 9 February. They were repulsed, and the Swedes left one of their assault bridges behind, which the Danes captured and measured. They found that the Swedish assault bridges were 36 feet long, and thus they realised that they could render these bridges useless by making the ice free parts of the moats wider than that.
The moats and the beaches had been kept free of ice, and now the ice free zones were widened to 44 feet with the help from 600 Dutch marines. The ice was thick, and the work was done in heavy snowfall from 4 o'clock in the afternoon till evening on the 10 February.
Spies reported that the Swedish army had moved from their camp, Carlstad, at Brønshøj and had taken up positions behind Valby Hill, and when the Swedes began their assault about midnight the same evening, they met with heavy resistance.
The main assault were made against Christianshavn and Vestervold, but the chopped-up ice and the massed weaponry on the wall made the densely packed attackers pay a horrific toll in lives. Still, they fought their way to the top of the wall, and fierce hand-to-hand fighting broke out.
When the Swedes realised that the assault on the Western part of the wall were in trouble, the choice was made to make a supporting attack at Østerport. The Swedes got very close to Nyboder and were in the process of crossing the moat, when they fell victim to a well-conducted ambush, and they withdrew with heavy losses.
At about five in the morning the Swedes gave up and retreated. They had taken severe losses. Before the walls 600 bodies were counted, and many more had perished in the ice-cold water, and were never found. On top of that there were many wounded. The Danes had only suffered about thirty dead.
Reference: Article in Danish on Wikipedia