Norwegian fortresses or fortifications have been constructed from some of the earliest recorded periods, down through the 20th century. The geography and topography of glacially carved, mountainous Norway constrain both the sea and the land routes which an aggressor must follow. Natural strong-points, such as rock outcroppings at Halden, Tønsberg and Trondheim make excellent bases for fortification (i.e., natural fortresses).
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.