Germany was able to take the Belgian forts around Liege relatively quickly when compared to the stagnant Western front for most of the rest of the war. What enabled the Germans to have success here when they couldn't break through later in the war? Was it simply that the forts provided static targets for German artillery?
The forts turned out to have two significant weaknesses. First, they were made of un- reinforced concrete, so that German artillery was able to slowly pound them into surrendering. Second, they had no large guns of their own: (the ones they had ordered from Krupp were not delivered- of course, Krupp was a German company). If they had had large artillery to respond to the German barrage, and their forts had been better hardened, the Germans would have had a much harder time, as their own big guns were comparatively unprotected.
Also, in this early stage of the war the fighting was far more fluid: the trenches hadn't been dug. Battle lines were constantly changing. Later, the battle lines around the trench systems would be quite well manned , supplied and supported by reserve troops, so that any temporary territorial gains by one side would be quite hard to secure against a counterattack by the other. Whether the Germans could have been stopped with better Belgian/British strategy and tactics ( or if the French had jumped to Belgium's aid) despite the loss of the forts, is another question.