Which Belgian city held out for 12 days against the Germans' Schlieffen Plan advance, thus forcing first, alterations to the plan and secondly, its abandonment?

1914 saw the end of the type of moving warfare begun by the Germans in their invasion of Belgium in August.  By the end of the year trench warfare had begun, with the trench line extending from the Channel coast to the Swiss frontier.  Thus the battles of 1915 on the Western Front were attempts by both sides to break out of this.