Thomas Wilhelm, A Military Dictionary and Gazetteer
“ Defensive fortification is the art of surrounding a place by works so disposed as to render it capable of a lasting defense against a besieging army. Offensive fortification comprehends the various works employed in conducting a siege. Natural fortification consists of those obstacles which nature affords to retard the progress of an enemy; such as woods, deep ravines, rocks, marshes, etc. Artificial fortification is that which is raised by human ingenuity to aid the natural advantages of the ground, or supply its deficiencies. ”
