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

Military History Podcast is sponsored by Armchair General Magazine