Some WW1 battle were fought for control of older fortifications, like the Battle of Verdun and the Siege of Przemyśl.
Did these fortifications performed as expected? Were they much better than field fortifications? Are the fortification of the inter-war period, like the Maginot Line, based on them or on field fortifications?
I believe Clayton Donnell wrote a book a few years back on WWI fortifications on the Meuse (looks like this is it here). It dealt specifically with the expensive Belgian project to attempt (in vain) to prevent the next French/German war being fought through their country by building a chain of forts in the 1880's. That put them a few decades out of date by WWI.
The moral of the story, to attempt poorly to sum things up, was that the forts were terribly outclassed by modern weaponry (German 42 cm howitzers were able to destroy the concrete reinforcements) and held off the Germans for roughly a week instead of the month they were designed for, but that week was sufficient to allow mobilization to occur to eventually hold the Germans back. In other words, they did not fulfill their paper mission, but they contributed in no small part to eventual success.
.....not really at all.
The root issue was that the fort in a traditional sense really only exposed forces to enemy artillery, and was expensive to boot. This is the entire reason why trenches were so entirely important in WW1- they were inexpensive and offered real protection. A fortification just gave you guidelines on where to drop shells, but a properly dug trench told relatively little while maximizing protection. Artillery shells in WW1 operated on a level of magnitude where if it landed in a fort, it hit something important, and if it didn't, the crap it kicked up probably hit someone.
I guess you could argue that the fact that they bought time meant the forts fulfilled their purpose but for effort (man hours, money, any metric you pick) in versus yield out they didn't really prove themselves worthy.
It really wouldn't be until WW2 or so, and period-contemporary construction realized that the usage of densely populated re-bar reinforced concrete was a colossal pain to deal with. Some Nazi structures were so difficult to destroy that even in peace time, the occupying forces just caved (no pun intended, I swear) and buried them. Urban explorers still like poking around some of them today in Germany.