Is it ensure, insure, or assure?
c.1400, "siege tower" (late 13c. in Anglo-Latin with a sense "bell tower"), from Old North French berfroi "movable siege tower" (Modern French beffroi), from Middle High German bercfrit "protecting shelter," literally "that which watches over peace," from bergen "to protect" (see bury) + frid "peace." Originally a wooden siege tower on wheels ("free" to move); it came to be used for chime towers (mid-15c.), which at first often were detached from church buildings (as the Campanile on Plaza San Marco in Venice). Spelling altered by dissimilation or by association with bell (n.).