O.E. wæcce "a watching," from wæccan (see watch
(v.)). Sense of "sentinel" is recorded from c.1300; that of "person or group officially patroling a town (esp. at night) to keep order, etc." is first recorded 1539. Meaning "period of time in which a division of
a ship's crew remains on deck" is from 1585. Sense of "period into which a night was divided in ancient times" translates L. vigilia, Gk. phylake, Heb. ashmoreth.
"The Hebrews divided the night into three watches, the Greeks usually into four (sometimes five), the Romans (followed by the Jews in New Testament times) into four." [OED]
The meaning "small timepiece" is from 1588, developing from that of "a clock to wake up sleepers" (1440). Watchmaker is recorded from 1630; watchtower is attested from 1544.