Impenetrable

Though they are not the only great walls of history, the following four certainly rank high:

• Great Wall (Started around 200BC): This wall, which is 25 feet high and 25 feet thick, was built to keep Mongol Invaders out of China. It was equipped with many watchtowers and smoke signaling systems. However, this Wonder of the Medieval World is deteriorating due to weather and American corporate influence. Recently, the wall was cleared by a skateboarder (The first time a non-motorized vehicle cleared the Wall).
• Hadrians Wall (Started around 100AD): This wall, which was only 6 yards high, spanned across the entire width of Britain. It was used by the Romans to defend against Scottish tribes. The wall was supplanted by Roman garrisons and forts.
• Maginot Line (Built from 1930-1935): This line of forts was created along the Franco-German border, in order to stop an expected German offensive to the West. However, in WWII, the Germans simply went around the line via the Low Countries. Therefore, the term Maginot is associated with "something that is relied on but fails".
• Atlantic Wall (Built during WWII): This line of defenses was created by the Germans along the Atlantic Coasts of Norway and France. It was intended to deter an Allied amphibious invasion of Fortress Europe. However, the Allies still continued with their attack and were able to break through at Normandy. However, this massive collection of defensive tools greatly slowed the Allied advance.

For more information, read:
http://www.greatwallofchina.cn/
http://www.hadrians-wall.org/
http://www.maginot-line.com/
http://search.eb.com/dday/browse?browseId=237176
http://www.atlantikwall.net/
The Reader's Digest Illustrated History of WWII

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