From Classic Encyclopedia 1911
BOULEVARD (a Fr. word, earlier boulevart, from Dutch or Ger. Bollwerk, cf. Eng. "bulwark"), originally, in fortification, an earthwork with a broad platform for artillery. It came into use owing to the width of the gangways in medieval walls being insufficient for the mounting of artillery thereon. The boulevard or bulwark was usually an earthen outwork mounting artillery, and so placed in advance as to prevent the guns of a besieger from battering the foot of the main walls. It was as a rule circular. Semicircular demi-boulevards were often constructed round the bases of the old masonry towers with the same object. In modern times the word is most frequently used to denote a promenade laid out on the site of a former fortification, and, by analogy, a broad avenue in a town planted with rows of trees.