 The bunkers were of different types, but all of them were large, able to house at least one anti-tank gun, several heavy machine guns and a crew of up to 20 soldiers.The walls and ceilings were made of reinforced concrete mixed with dense porphyry rocks, which added to their impregnability.All bunkers in the area had code-names starting with the letter W. Despite the plans, in less than two months only five bunkers were more or less ready.