Overview of the five big battles of Ypres during World War I (Map Ypres Salient: Copyright T & V Holt Associates 2002 (Major & Mrs. Holtís Battle Map Of the Ypres Salient)).

A large amount of ground won in Third Ypres was later again lost but the British kept ground at Pilckem because of the strategic importance of the hilltop location. Trenches and other infrastructure were maintained until October 1918; some trenches were even enlarged. On 8 August that same year the allied forces broke permanently through the German lines. The war ended on 11 November 1918.
[More information about the Battles of Ypres can be found in GILES, 1979]