Walls prevent movement of any ground based units. Only flying units may cross Walls, although friendly units may pass through Walls at Gates. Wherever humans developed agriculture and created surpluses of food, there were less fortunate or prudent neighbors who wanted those riches and were willing to fight for them. The bounty of agriculture thus needed protection almost immediately. The earliest and most simple defensive structure was the wall. The earliest towns so far discovered are walled for defense. Strong walls prevented easy movement into a town and provided cover if enemies attacked. Walls were made with whatever materials were handy and useful. Wall building has continued for thousands of years, and is still carried on today. "One will build the Walls-of-the-Ruler to bar Asiatics from entering Egypt; They shall beg water as supplicants so as to let their cattle drink. Then Order will return to its seat while Chaos is driven away." -- the Prophesies of Neferti
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