Hedgehog
Designed as a general purpose obstacle for the expected sea borne assault on the Northern French coast, hedgehogs generally consisted of three steel beams that were crossed at their centers and welded together.
This simple yet effective design functioned by preventing the Allied attackers a clear and easy path to the beaches. Even if the incoming landing craft could see and avoid the hedgehogs hidden underwater (most obstacles were placed between the low and high waterlines), those that were above the waterline hindered landing operations and clogged the beach.
Although the hedgehogs did provide the attacking forces with a small amount of cover from German fire, this "advantage" was insignificant compared to the problems created by their presence.
Ironically, the steel beams of the hedgehogs (and other steel-based obstacles) were later used to outfit Allied tanks with devices used to cut through the thick hedgerows that were slowing down the Allied drive into France.
