Fortifications evolved to accommodate the offensive threat which they guard against. Early castles provided a strong defense against the attack of the day, and were normally taken by duplicity or siege. In the age of black powder, cannon allowed breaching of the fortress walls and subsequent taking by storm. As a result fortresses changed form, now incorporating design features like the bastion, ravelin, and glacis to allow cannon within the fortress to be effective while protecting the walls and defenders from external attack. This evolution of technology continued into the 20th century as weaponry continued to evolve.
In 1600 Denmark controlled virtually all land bordering on the Skagerrak, Kattegat, Store Bælt and the restricted Sound (Øresund). The current Swedish provinces of Skåne and Halland were Danish and the province of Båhuslen was then Norwegian (as they had been for all recorded history). All powers interested in Baltic trade or otherwise forced to pass through waters controlled by Denmark had a strong interest in breaking Denmark’s control and lifting the “Sound dues” Denmark levied for passage through the Øresund. Hence the naval trading powers, particularly Holland and England, contributed to the Northern unrest of the period.
And the larger political balance in Europe can not be forgotten. As one example, the danger of French domination under Louis XIV resulted in a 1668 triple alliance of England, Holland and Sweden. This alliance worked to Sweden’s favor when treaties were negotiated.
The Struggle for Supremacy in the Baltic: 1600-1725 by Jill Lisk; Funk & Wagnalls, New York, 1967
The Northern Wars, 1558-1721 by Robert I. Frost; Longman, Harlow, England; 2000 ISBN 0-582-06429-5