early 15c., from Middle Dutch bulwerke or Middle High German bolwerc, probably from bole "plank, tree trunk" (from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell;" see bole) + werc "work" (see work (n.)). Figurative sense is from 1570s.
mural towers, bastions, were introduced by king Uzziah (2 Chr. 26:15; Zeph. 1:16; Ps. 48:13; Isa. 26:1). There are five Hebrew words so rendered in the Authorized Version, but the same word is also variously rendered.