Germany raised an opposite Siegfried Line.
Belgium's refusal to extend its fortifications on the german border and the bad reception of an intent to fortify the line on the Franco-Belgian border meant there was a gaping hole in the line. This period saw a short moment of cold relationships between France and Britain over the possibility that the Entente Cordiale might not last, and fortifying the franco-belgian border was perceived as tantamount to an act of war, when Belgium was conceived as a buffer to avoid french imperialism in the first place (the only defensible belgian border in 1836 was the french one, as the northern fortifications all remained in the Netherlands).
It accomplished its purpose in war: nobody invaded through the Rhine, though. Also it made Patton look like a buffoon for a short while (siege of Metz: "I will have it in a week" (paraphrasing)).