bulwark (n.)

early 15c., "a fortification outside a city wall or gate; a rampart, barricade," from Middle Dutch bulwerke or Middle High German bolwerc, probably [Skeat] from bole "plank, tree trunk" (from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell") + werc "work" (see work (n.)). Thus "bole-work," a construction of logs. Figurative sense "means of defense or security" is from mid-15c. A doublet of boulevard.