"tower containing a clock," usually a large one with dials exposed on all four sides, 1757, from clock (n.1) + tower (n.). Older words for this were clocher (14c., from Old French), belfry.
Entries linking to clock-tower
clock n.1
"machine to measure and indicate time mechanically" (since late 1940s also electronically), late 14c., clokke, originally "clock with bells," probably from Middle Dutch clocke (Dutch klok) "a clock," from Old North French cloque (Old French cloke, Modern French cloche "a bell"), from Medieval Latin clocca "bell," which probably is from Celtic (compare Old Irish clocc, Welsh cloch, Manx clagg "a bell") and spread by Irish missionaries (unless the Celtic words are from Latin). Ultimately of imitative origin.
Wherever it actually arose, it was prob. echoic, imitating the rattling made by the early handbells of sheet-iron and quadrilateral shape, rather than the ringing of the cast circular bells of later date. [OED]
Replaced Old English dægmæl, from dæg "day" + mæl "measure, mark" (see meal (n.1)). The Latin word was horologium (source of French horologe, Spanish reloj, Italian oriolo, orologio); the Greeks used a water-clock (klepsydra, literally "water thief;" see clepsydra).
The image of put (or set) the clock back "return to an earlier state or system" is from 1862. Round-the-clock (adj.) is from 1943, originally in reference to air raids. To have a face that would stop a clock "be very ugly" is from 1886. (Variations from c. 1890 include break a mirror, kill chickens.)
I remember I remember
That boarding house forlorn,
The little window where the smell
Of hash came in the morn.
I mind the broken looking-glass,
The mattress like a rock,
The servant-girl from County Clare,
Whose face would stop a clock.
[... etc.; The Insurance Journal, January 1886]
tower n.1
Old English torr "tower, watchtower," from Latin turris "a tower, citadel, high structure" (also source of Old French tor, 11c., Modern French tour; Spanish, Italian torre "tower"), possibly from a pre-Indo-European Mediterranean language. Meaning "lofty pile or mass" is recorded from mid-14c. Also borrowed separately 13c. as tour, from Old French tur; the modern spelling (1520s) represents a merger of the two forms.
belfry n.
c. 1400, "wooden siege tower on wheels" (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," from Proto-Germanic compound *berg-frithu, literally "high place of security," or that which watches over peace." From bergen "to protect" (from PIE root *bhergh- (1) "to hide, protect") or [Watkins] *bergaz "mountain, high place" (from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts) + *frithu- "peace; personal security" (see affray).
The etymological meaning was forgotten, which led to folk-etymologies and a great diversity of spellings. It came to be used for bell towers (mid-15c.), which at first often were detached from church buildings (as the "Leaning Tower" of Pisa and the Campanile on Plaza San Marco in Venice), and the spelling was altered by dissimilation or by association with bell (n.).