Young Black Haired Girl Shows Off At The Fortress

The terrain that was being defended was of primary importance because open terrain that tanks could move over quickly made possible rapid advances into the defenders' rear areas that were very dangerous to the defenders. Thus such terrain had to be defended at all cost.
In addition, since in theory the defensive line only had to hold out long enough for mobile reserves to reinforce it, terrain that did not permit rapid advance could be held more weakly because the enemy's advance into it would be slower, giving the defenders more time to reinforce that point in the line. For example, the battle of the Hurtgen Forest in Germany during the closing stages of World War II is an excellent example of how impassable terrain could be used to the defenders' advantage.
After World War II, ICBMs capable of reaching much of the way around the world (and rockets capable of reaching the Moon and other nearby celestial bodies) were developed, and so speed became an essential characteristic of the strongest militaries and defenses. Missile silos were developed, so missiles could be fired from the middle of a country and hit cities and targets in another country, and airplanes (and air carriers) became major defenses and offensive weapons (leading to an expansion of the use of airports and airstrips as fortifications). Mobile defenses could be had underwater, too, in the form of nuclear submarines capable of firing missiles. Some bunkers in the mid to late 20th century came to be buried deep inside mountains and prominent rocks, such as Gibraltar and Cheyenne Mountain. On the ground itself, minefields have been used as hidden defences in modern warfare, often remaining long after the wars that have produced them have ended.
Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between potentially hostile militaries.