crenel (n.)

"small, narrow opening, crevice," mid-15c., possibly from a diminutive of Old French cran, cren "a notch, a hole, a cut, fissure" (14c.), from crener "to notch, split," from Medieval Latin crenare, which is possibly from Latin cernere "to separate, sift" (from PIE root *krei- "to sieve"). Compare Old French crene "notched." But OED casts doubt on this derivation. Related: Crannied (mid-15c.).

"to furnish with a battlement, render defensible," 1823, from crenel + -ate (2). Sometimes also crenellate; the double -l- seems to be from a presumed Latin *crenella as a diminutive of crena. Related: Crenelated, also crenellated; crenelation, crenellation (1849).

The Middle English verb for this was carnel (early 14c.), from the noun in Middle English and from Old French crenelé, from crenel.