Jurjen,
The difference between a pillbox and a bunker. Well in short, a pillbox is a small bunker, without interior, so no rooms, walls, storage or anything. It is just a concrete hut to protect a few soldiers or MG nest who would be regularly replaced. A bunker is is a larger construction, with storage rooms for ammo anf supplies, sleeping quarters etc, etc. These can house anything from a rifle platoon to a brigade for a prolonged time. I think what you have build classes as a pillbox as there seem to be no internal walls or such. As you have a wall at the back which extends around the entrance, a door would not neccessarily be fitted. I do however think that there should be a firing or observation slit on the other side of the bunker as there is now only one field of fire. Unless the bunker would be part of a intricate system of bunkers (such as the Siegfried Line) with overlapping fields of fire, a bunker or pillbox would never be blind/defenceless on one or more sides.
Groeten
Henk