Bunker
Pillboxes were used a lot during World War I when defence in depth was being used.  Pillboxes are most often found camouflaged so that they are harder to spot.  They can also be part of a trench system, where the pillbox is a firing step that has been built to take grenade blasts and smaller mortar fire.  Pillboxes were also built to help protect strategic structures such as bridges or jetties.