During World War I, the zigzag pattern of trenches on the front lines was designed to

IT was to protect troops from enemy fire

During WWI, the trenches were designed in zig-zag patterns, so that if the enemy ever got into the trench, and they started firing the bullets wouldn't be able to go straight down the trench killing all the soldiers instead they would be stopped and hit off the turns and sharp curves. It was a clever defence mechanism.