by Erik Ettrup

"R" bunker = Standarized bunker for the heer
"M" bunker = Standarized bunker for the Kriegsmarine
"L" bunker = Standarized bunker for the Luftwaffe
"FL","V" & "S" =Sub-types of the "M" bunkers depending on its use
"Vf" bunker = (verstärkt Feldmässig) Light version of the ones mentoned above.
"SK" bunker = (Sonderkonstruktion) Special construction. Differs from the ones mentoned above.

After 1935,German engineers once again started making plans for a strong forticication line to strenghten their eastern and western frontiers.
In order to speed up and economize the construction there were made a handfull of standard designs, called "Regelbauten". The engineers soon found that most of these designs were unfit for specific locations. During the construction of the "West-wall", over 200 sub-types were designed. 
Even Adolf Hitler made his own "Regelbau" based on his experiences from WW1. This was a small bunker which had a machinegun and a flamethrower.

After "The battle of Britain" was lost, a massive plan was launced. It was decided to build a fortification line going all the way from southern France trough the low-countries, Denmark, Norway and into Petsamo in Finland.
The fortification was given the name "Atlantikwall". Coastal artillery was placed so close to each other, that no ship could reach shore anywhere, without being in distance for atleast one battery.Each battery was surounded
by strongpoints in order to protect the batteries. The old 100 series for the westwall, was replaced with the newer 2-500 series, which was more adapted to coastal defence.

The weapons
Most batteries were equipped with captured field artillery. The most common calibres were between 10 and 20 Cm, But in some batteries, huge navalturrets was mounted. For example the 28cm main turrets from Gneisenau, the 40,6cm turrets from the planned "H" battleship. In a few occations, torpedobatteries as well.
The main close defence weapons were various anti-tank weapons, machineguns, mortars and flamethrowers.

Bunkers for the Heer
The first really standardised system came with the introduction of the 100 series in 1939.
Their designs were divided in two strenghts, Baustärke A & B (2 & 3,5M concrete).
The 100 series were designed for machineguns, armoured cupolas and the Czechoslovak 4,7 cm Festungspak.
During the construction of the "Atlantik-wall", the 100 series were divided into 2 new series called 400 and 500.
Late 1942 The 600 series were launched. This was mainly for coastal artillery, but some of them got popular as regular personell/radio bunkers all over the Atlantikwall.
As the Heer had the absolutely widest spectre of bunkertypes (Including coastal artillery) The R bunkers were often used by personell from both the Kriegsmarine and Luftwaffe
More info on the Bunkers of the Heer

Bunkers of the Kriegsmarine
The first standardised navy bunkers were built in the countries which were invaded in 1939-40.
They had several own bunkers which was given the main designation "M" instead of "R".
Most of the "M" bunkers are compareable to the Heer versions.
They practically have the same functions, but yet it was decided to have a complete series for the navy!
The "M" bunkers has a lot of their shape taken from warships, and most of them are built over several floors.
Many of them are infact a direct reflection of a warships interior! Often lots of steelplates and armoured parts
are used within the "SK" bunkers, giving them a rather strange look.
The "M" system were also divided into 4 classes, M, Fl, S, and V
-M & S: Function within medium or heavy coastal artillery
-Fl: Function within air defence (only for Marine-Flak abteilungen)
-V: Hq's, dressing stations and radar's
In order to protect the artillery, massive casemates were built over the of the largest was used
for the 38 cm naval turrets. To tell a little of the size, it was 35x25x10 meters, and it took 6000 qubic meters
of concrete to build it!
More info on the Bunkers of the Kriegsmarine

Bunkers of the Luftwaffe
The first Luftwaffe bunkers were built in a paralell line to the Westwall, called "Luftverteidungszone West"
In the beginning they used the old 100 series, but it was quicly realized that they needed their own designs.
This lead to a series of ca 100 variations and got the designation "L". All "L" bunkers for Flak have underground ammo and personell shelters, with gun emlacements on the roof. The others are mainly for command posts and radars.
More info on the Bunkers of the Luftwaffe

General information
On top of this massive labyrint of designs, even more were added.
Again to lower the construction time and costs, it was decided that bunkers not lying exposed to direct fire,
could be built as a light version (Verstärkt feldmässig, or VF)
The VF bunkers are found within all designs of the entire Wehrmacht.
The OT and Bau-pioneers which were actually building the bunkers, often made their own designs called
"Sonderkonstruktion or SK. SK bunkers were usually built for "one of a kind" equipment.
Examples of SK bunkers could be for Submarines, S-boats, command posts and the "Führer bunker".
Some of the regular R,L or M bunkers may also have a little different interior depending on the purpose
it was built for. It would then be given the designation SK after its R number (eg. R-608 SK)

Bunkers can be divided in 3 main groups

The defence bunkers had 1 or 2 main weapons. The weaponry was not standarized at all. The ones used most is the Mg-34, Mg-42, 3,7 cm Pak, 4,7cm Pak, 5cm KWK, 7,5 cm pak and flamethrowers.
In addition, most Flak had the possobillity to direct fire agaist ground targets.
It also seems like loads of captured weapons were used in static defence.
Most of these bunkers are built as an independent battle unit. The Standarized structures were allways built gas-tight, and with the possibility to be "sealed" in case of an attack.
Most often a manually operated ventilation system was added inside. Working togeter with filters and overpressure valves, these bunkers were able to withstand a gas attack.
All ventilation shafts are "S" shaped and closed with several nets inside, making it impossible to roll a grenade inside.
When approaching a bunker you often see the "Eingangsverteidigungs scharte" which means something like "Entrance defence hatch". The hatch gives a full view of the entrance, making it possible to see who's coming.
You enter the bunker trough the "Kampfraum" (Battleroom). Here is the main door protected with another "Eingangsverteidigungs scharte" Which has view over the entire battleroom. The main door is always divided in 2 parts, upper and lower. This makes it possible to open, even if the battleroom is filled with debris. The maindoor leads you into the "Gas lock" wich is a little room where the air can be filtrated before entering. Inside the bunker there are several rooms depending on it's purpose.
Each room is divided with armoured doors, so even if the enemy got trough the battleroom, the bunker is not lost....yet.
There are sleeping rooms, radio room, generator room and ventilation room to mention a few.
Rooms containing machineguns also has their own ventilation which filtrates the poisonus gases from gunpowder.
Large bunkers often have their own power-station inside. usually diesel engines running a generator.
In smaller structures, the bunker gets electricity from outside, but will then have battery backup for the radio equipment.
Bunkers with only one entrance often has a emergency exit. This is constructed in a way that makes it impossible to enter from the outside.
First you open a small armoured door. Then you slide out 15-20 "Iron rails" behind the rails there is a brickwall which must be removed. Aftre removing the bricks you have to dig to the surface(2-6 meters!)

Manual observation was very important. Most of the artillery fire was directed by firecontrolposts with huge rangefinders.
In order to triangulate the distance to the target, each battery had 2 "Peilstande". The main firecontrol was usually a large structure which housed the main rangefider, the battery officers and radio station.
The luftwaffe had their own airraid warning bunkers called "FluWa". These were often placed near these fortresses and had direct contact with the surrounding airfields.
A wide spectre of observationbunkers were made, both for coastal and regular artillery, often with armoured copulars.
In addition to manual observation, there were radars.Since the radars were very large and voulnerable, they were often placed apart from the rest of the bunkers.
Both Kriegsmarine and Luftwaffe had their own design for each radar type.

The support bunkers were placed behind the frontline of the batteries. There were own designs for every piece of
equipment, even field kitchens! Ammunition bunkers, personell shelters, headquarters, dressing stations and communication bunkers could be examples of this.