All you ever wanted to know about fortifications but never asked by Walls and Gardens (project)
Sponsored by various UK and European local governments "understand your local fortifications".
An introduction to the history of fortifications and their building technology.
Think (Not Free) Down By the Sally Gardens perhaps ?
http://www.amazon.com/gp/product/B00S1IAGQ6/
http://www.amazon.co.uk/gp/product/B00S1IAGQ6/
http://www.amazon.ca/gp/product/B00S1IAGQ6/
Also free in French and Dutch
Tout ce que vous avez toujours voulu savoir sur les fortifications sans oser le demander (French Edition)
Alles wat je wilde weten over vestingen maar nooit durfde te vragen (Dutch Edition)