As the campaign in Italy continued, the rough terrain prevented fast movement and proved ideal for defence, the Allies continued to push the Germans northwards through the rest of the year. The German prepared defensive line called the Winter Line (parts of which were called the Gustav Line) proved a major obstacle to the Allies at the end of 1943, halting the advance. Operation Shingle, an amphibious assault at Anzio behind the line was intended to break it, but did not have the desired effect. The line was eventually broken by frontal assault at the Fourth Battle of Monte Cassino in the Spring of 1944 and Rome was captured in June.