The beach obstacles already described are the lesser installations of the elaborate system of coastal fortifications which the Germans have constructed. The backbone of the defenses consists of reinforced concrete works, including units of many different types and sizes. In some places the defenses are rather light—barbed-wire fences, minefields, machine guns, or antiaircraft guns in small numbers—but in all the strategic areas the Germans have taken advantage of the great deal of time at their disposal to lay out their defenses carefully and in depth, taking the maximum advantage of terrain features.

In some regions the Germans, before planning new defenses or improving old ones, held amphibious maneuvers to discover vulnerable approaches. Then the plans for fortifications were made in the light of lessons learned. They also profited from lessons drawn from actual operations, such as the Dieppe and various commando raids.

The Germans have used reinforced concrete lavishly in constructing forts, underground shelters, pillboxes, dumps, and emplacements for light and heavy weapons and for troops, as well as waterfront shelters for submarines and motor torpedo boats. Maximum use has also been made of existing French, Belgian, and Dutch coastal fortifications, which have been strengthened by the addition of armor plate, guns, and other matériel from the Maginot Line and other interior fortified positions, by resiting of emplacements, and by additional concrete construction.

The major part of this work has been performed by the Organisation Todt (O.T.), a semimilitary agency that was formed primarily to build Germany's West Wall opposite the Maginot Line before the opening of the war. Organized into labor battalions, this agency employs a large number of locally hired and conscripted workers in carrying out its projects.

The following figures, which give average dimensions, are offered as a general guide to the strength of Todt-built concrete works on the coast:

	Observation posts	Personnel shelters	Ammunition magazines
Thickness of walls	3 to 5 feet	2 1/2 to 3 feet	3 feet.
Thickness of roofs	6 1/2 feet	3 to 5 feet	3 feet.
Depth of soil on underground works.	6 1/2 feet	0 to 3 feet	0 to 4 feet.

This section is devoted to descriptions of various unit-types of German field and permanent fortifications which are common on the Atlantic coast. They are discussed individually, and in detail if data is available, for the purpose of showing their characteristics and providing a basis for estimating their structural strength and weaknesses. It must be borne in mind, however, that each unit is only one component of a "hedgehog," or well-calculated system of all-around defense, and that the individual unit contributes its own fire power to, and derives support from, the other elements of the system. (See par. 18, below.)