People devise fortresses because in the past, the cost of defense is cheaper than the cost of attack. The defender builds a very high and thick stone/brick wall (certainly not cheap, but the cost is spread out throughout time)and station X amount of people on top of it, and the attacker will need at least 5 times the amount of people and equipment (immediate and present cost) to take it (just look at how many people is scurrying up the ladder, and how many is being killed on the ladder by arrow, hot water, burning log in the movies).
It is not until recently, when the advancement in long range artillery technology and modern mass-production industry that reversed the situation. Even the toughest wall can't hold against the repeat bombardment of cheaper, and more powerful artillery rounds that can be sent off the factory floor around the clock.
Also, smart generals from all era and places in history always try to dismantle a fortress without turning it into a slug match. <PERSON> has said that "Attack a forturess head on is the worst strategy", and <PERSON> (I forget whether it is him) has said something similar to "If you barricade yourself in an impenentrable fortress, your enemy will seek other ways to get to you." A fortress must have self-sufficient food, water, medical, and basic manufacture ability to resist a seige. Special forces has been used to sneak into the fortification to destroy these vital facilities. 121
The larger the fortress, the more unfamiliar the inhabitants are to each other. This allows spys and sabotager to sneak in.
To make fortress prominent again, one of these factors must be eliminated. Perhaps this is after a nuclear war that decimated most of the factories around the world. The fortresses are the remaining factories, who needs to be defended with everything the defender has and who can pump out ammunition and other war machine as fast as possible while the attackers must use numerical advantage. Some new methods can be used to detect spys and sabotageurs, such as bio-integrated microchips or just a guard dog/bear/mechanical dragonfly. 968