A lesson learned in the trenches of WW1
In the aftermath of the "Great War" of 1914-18, the incredible horrors and suffering experienced on both sides by the soldiers of trench warfare echoed throughout Europe. For a great many people, the sacrifices endured by the men and the blatant disregard for the life of their troops, displayed by ambitious (and in some cases rather incompetent) commanders led to a disgust for war in general and the rather naive assumption that this conflict had been "the war to end all wars". This view prevailed throughout many states in Europe and in the USA. As history would demonstrate, it also nourished that spirit of defeatism and policy of appeasement with the allied that paved the way for the next war, brought about partly by the harsh peace terms imposed upon Germany (resulting in poverty and dire need amongst common people, combined with shattered national pride), the crumbling of Imperial reign and subsequent ineffective attempts to establish democracy and eventually the rise of a totalitarian regime. 

All this can be read in any history book and is outside the scope of this page. But this atmosphere of mutual distrust also was the spark that started the building of huge fortresses in Europe. 

The belief in fixed fortifications
France and Belgium may have shared the desperate hope for everlasting peace, but decided nonetheless to back it up by constructing mighty defense installations along their borders; the "impregnable" Fort Eben-Emael in Belgium and the mighty Maginot Line in France, multi-storey marvels of technology and sophistication in their time. The objective was to shield soldiers best possible while deterring any aggressor (Germany) from attacking.

In Germany they chose a slightly different approach when constructing the opponent to the French border fortifications. They were equally eager to avoid the perils of trench warfare and improve the conditions for the fighting men, but they deliberately did not want to let soldiers succumb to the docile existence and complacent life of the fortress inhabitant. Their response was to design fortifications based on individual bunkers connected with trench work. Troops could live in relative comfort, withstand and survive aerial bombardment or heavy shelling in their bunkers, but had to stay vigilant as they must leave the safety of these positions to fight.  

The Wehrmacht operated with three levels of fortification;

1. Feltmessige Anlage (Field-type constructions)
Mainly constructed from locally available materials; lumber, sand, dirt, turf and rocks. Concrete was also used if available. A wall and ceiling thickness of 40 to 60 cm was sought and a compound construction where e.g. a double timber wall was lined with rocks was often used. The floor would be approx. 20 cm thick. The construction was intended to be able to stop small-arms fire, shrapnel and light debris.  

2. Verstärkt Feltmessig or "Vf" (Reinforced field-type constructions)
These were "real bunkers", carried out as concrete or ferro-concrete constructions (concrete reinforced with steel bars) with wall and ceiling up to 100 cm (three feet) thick and a floor up to 40 cm. The construction was designed to protect its inhabitants from the effects of a 50 kg aerial bomb dropped on the roof or a direct wall hit from a 105 mm artillery shell. Some of these designs became designated types in the Bauform catalogue.

3. Ständige Anlage or "St" (Permanent constructions)
The Regelbau concept embraced only permanent constructions which came in multiple "baustärke" (building-strengths, referring to dimensions). Most common is Baustärke B (Build strength B) featuring ceiling and wall dimensions of min. 200 cm (6.5 feet) and a floor of 80 cm. This impressive construction was sufficient to protect the soldiers within from the effects of a half-ton aerial bomb on the roof (max. conceivable size at the time of design), or a hit from a 220 mm artillery shell. If local conditions required an even sturdier construction, you could go to Baustärke A, comprising walls and ceiling of 350 cm (almost 12 feet) and capable of withstanding the heaviest artillery of the day and aerial bombs up to 1000 kg (one metric ton). (For comparison, Hitler's bunker in Berlin boasted 450 cm - more than 15 feet - worth of concrete protection, and the roof of his private bunker in the "Wolfschanze" (Wolfs Lair) in Rastenburg, East Prussia, allegedly sported a stunning 1400 cm or some 46 feet).

Furthermore, a ständig bunker was, if used for habitation or as a fighting post, secured against gas warfare, had it's own air-cleaning plant, telephone, radio, heating, lighting and amenities.  Some hard their own well for water supply. The "St" bunkers were designed to suit a variety of purposes, each model with its own designated number and eventually grew into becoming the Regelbau concept. 

Elements of the Regelbau concept
The Regelbau concept formed the backbone of the Atlantic Wall. Freely translated, Regelbau means something down the line of "Standard Build", and standardization was truly the name of the game with close to 700 individual bunker types in the book at the end of the war, each with a specific task and identified by a unique number. This approach to fortifications allowed the contractor, Organisation Todt (OT) quickly to adapt any stronghold or fortified area to meet local conditions, strategic, tactical and geographical and to expand existing facilities without compromising the integrity.

The comparison to a standard house is only valid however, as to the repeated use of standardized elements. Apart from that a Regelbau bunker looks nothing like your everyday gazebo, and most prominently is the huge dimensions of walls and roof.

Apart from wall thickness, Regelbau bunkers share a series of similar features:

Entrance. If at all possible, the entrance ends in a 90° turn, covered by an embrasure for small-arms firing, protected by a 30 mm steel plate. Fighting posts (where the crew remains inside during combat, as i.e. in a radar-bunker) are further protected by a flanking gun embrasure providing enfilade fire along the entrance side. This is referred to as the close-combat room.

Gutters. This is a later addition to the basic design. Keeping rain water from entering doorways, air intakes and gun embrasures, these gutters also guided burning flame-thrower fuel away. 

Armored air intake. Sturdy steel grills protected the air intakes 

Doors. The standard door is a 30 mm thick armored steel door of the stable-door type. It can have rounded or square corners. Some types have man holes. A rubber sealing round the rim of the door makes it gas-tight. The inner door in a Regelbau bunker are often thin steel doors, but can also be wooden doors. 

Room dividers. The size of a Regelbau bunker as well as the number and dispersion of rooms is strictly determined by it's function. Only the required rooms are present and no room is larger tahn absolutely necessary. Internal walls could be cast in concrete or laid in bricks or even wood. 

Wall lining. In general, internal walls were lined with wood, partly because of the insulating effect, but also to minimize the effect of concrete blow-off caused by a direct hit on the outside. In later constructions, steel netting was applied in the casting process to improve the latter. 

Flooring. In some bunkers you will find a flooring of (usually brown) asphalt tiles. These served as vibration dampers to counter the effects of shelling and bombardment that otherwise cause severe bodily injury to the inhabitants such as factures and damage to internal organs. In many bunkers you will se a warning against just that painted on the walls.

Gas protection. War-gasses being one of the weapons of mass destruction of WW1, most Regelbau bunkers - including all habitats and fighting posts - were secured against chemical warfare with air-tight doors, an airlock, pressure valves, armored air intakes and a air-purification device that could be either electrically or manually driven. The device created a higher pressure inside the bunker thus effectively preventing gas to slip through gaps in e.g. gun embrasures etc. Internal pressure was aligned by the valves.

Close-combat room. This is an ancient detail, used since the medieval; the flanking position to provide enfilade fire along the walls of the "fortress". Usually the gun embrasure is shaped as an inverted step-pyramid and lined with wood to prevent ricochet bullets from entering the bunker.

Well. Some bunkers have their own well, either right outside or inside the bunker. Today this may just appear as a rather deep - and not necessarily covered - hole in the floor, so mind your step!

Antenna. In many bunkers a carving in the concrete wall just outside the armored door can be observed, and in some even a metal fixture has survived. The carving leads to a hole in the ceiling and is designed to lead a segmented, telescopic radio antenna through the roof of the bunker. 

Emergency exit. Bunkers with just one entrance often feature an emergency exit. Today this is most often just a hole in the wall, but you can be lucky, as the photo to the right demonstrates, to find an intact exit, complete with metal bars, brick wall and a gravel-filled stepladder tunnel.

Telephone. Used for communication between bunkers and HQ. Often bears the warning; "Achtung - Feind hort mit!" - Caution, the enemy is listening!

Voice-pipe. For communication between bunker an entrance or the look-out in the Tobruk (see below). A simple and reliable, non-electric system as it is also known on vintage ships. 

Heating. All bunkers where the crew had to stay for a prolonged period of time had heating. In larger facilities a form of central heating was quite often used, but in it's form, heating was provided by an oven, using coal or wood as fuel, and with or without a boiler plate for cooking. The chimney pipe was constructed with a trap, leading a hand grenade dumped into the chimney to the outside of the bunker.

Periscope. In some bunkers, e.g. crew bunkers and fighting posts, you will observe a circular, 5 inch aperture in the steel ceiling. This is designed for a retractable u-boat style periscope, enabling the inhabitants to take a 360° peek of the area outside before opening the door. As so much other bunker equipment, periscopes have long vanished from rural bunkers, but working specimens kan be found i.e. at Aalborg Forsvars- og Garnisionsmuseum and at Silkeborg Bunkermuseum. 

Toilet. Most often just a bucket with a seat, and placed in the airlock (most suitable, one might argue). In some cases a regular "outhouse" was constructed. Larger command bunkers had internal facilities. 

Warnings and other works of art. The lucky bunker explorer might occasionally stumble over inscriptions or pictures, written or painted on bunker walls. In most cases it will be instructions or warnings, but it may also be Nazi insignia, enflaming paroles of courage or even artistic attempts by homesick soldiers to depict sceneries from home or a loved one left behind. Take all the photos you want, but do not disturb or destroy these messages from a sinister past. 

Armored copulas. These are found on artillery observation posts, on certain mortar bunkers and in specialized machine-gun or anti-tank gun emplacements. The topic will receive special attention at a later stage. Not to be confused with tank-turret positions.

Outside Regelbau
The Bauform catalogue comprised a number of thin-walled concrete constructions, predominantly used as frontline fortifications to house defensive positions with machineguns, anti-tank weapons or mortars. In a few occasions also stationary flame-throwers were built in. The most well-known is undoubtedly the "Tobruk", named after the city in Libya. 

Tobruk. The Tobruk or "ringstellung" is basically a reinforced foxhole, some with a small, two-man habitat attached to it. The simplest version is named Bauform 201 or 58c, but a a variety of bunkers emerged from it. Tobruks are also an integral part of many larger bunkers, where they serve as observation posts and machinegun positions. 

Some similar types housed an 80 mm mortar (Bauform 69), some were furnished with armored turrets provided by obsolete Germen and French WW1 tanks (Bauform 67). This is not to be confused with armored cupolas as stated above. Where turret or cupola has been removed, you can easily tell the difference by the size of the hole, as cupolas in general have a somewhat larger diameter.

Sonderbau, special constructions
Constructions such as Hitler's bunkers, the FlAK towers of Berlin and Vienna, the huge u-boat pens of Trondheim or the French west coast harbors, V2 production sites La Coupole or Le Blockhaus in France or the night fighter control center Gyges in Karup, Denmark were all "Sonderbau".