Found a really interesting article in the archive of the Illustrated London News today - http://www.illustratedfirstworldwar.com/item/tanks-of-other-days-old-time-forts-on-wheels-iln0-1916-1021-0011-001/
It shows how tanks have evolved through the ages and how the principle of a protective war machine was thought up as long ago as the 5th Century.