Young Black Haired Girl Shows Off At The Fortress

The strength of the defensive line in an area varied according to how rapidly an attacking force could progress in the terrain that was being defended—both the terrain the defensive line was built on and the ground behind it that an attacker might hope to break out into. This was both for reasons of the strategic value of the ground, and its defensive value.
This was possible because while offensive tactics were focused on mobility, so were defensive tactics. The dug in defences consisted primarily of infantry and antitank guns. Defending tanks and tank destroyers would be concentrated in mobile "Fire Brigades" behind the defensive line. If a major offensive was launched against a point in the line, mobile reinforcements would be sent to reinforce that part of the line that was in danger of failing.
Thus the defensive line could be relatively thin because the bulk of the fighting power of the defenders was not concentrated in the line itself but rather in the mobile reserves. A notable exception to this rule is seen in the defensive lines at the Battle of Kursk during World War II, where German forces deliberately attacked into the strongest part of the Soviet defences seeking to crush them utterly.
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.