One important area during the war was the town of Ypres:

Ypres lay on the most direct route to the Channel ports of Calais and Dunkirk
If Germany were to catch these ports they would cut off British supplies
Ypres had to be defended to keep the British War effort going
The Ypres Salient (salient means ‘bulge’) was a vulnerable area of the Allied line
German forces occupied high ground overlooking the Salient, which made it easy to fire down at the British and American forces
These higher areas were well drained and made for better treatment and defence on the German front
British trenches were on low-lying ground that was easily waterlogged and flooded

Theatre of War - Ypres