bastion n.

"sew together loosely," c. 1400, from Old French bastir "build, construct, sew up (a garment), baste, make, prepare, arrange" (12c., Modern French bâtir "to build"), probably from Frankish or some other Germanic source, from Proto-Germanic *bastjan "join together with bast" (source also of Old High German besten; see bast).

14th century Paris fortress later used as a prison and destroyed by revolutionaries on July 14, 1789, as a symbol of royal despotism; French, literally "fortress, gate tower" (see bastion). Many French cities kept their medieval gate-towers as prisons after other fortifications were removed. The word was in Middle English in the "fortress" sense as bastel, bastyle.

"an indented parapet in fortifications," early 14c., from Old French bataillement, earlier bastillement "fortification," from bastillier "to fortify, to equip with battlements," from bastille "fortress, tower" (see bastion). The raised parts are cops or merlons; the indentations are embrasures or crenelles.