Your answer is Trenches
Because they allowed you to duck behind a wall so that the enemy could not just shoot you out into the open the had to rush you which allowed you to pick them off one at a time but it was still way harder for them to shoot you because they cant see your whole body they ca only see part of it. these trenches helped not get hit by gunfire but also to use it as a medic area because it wasn't out in the open.