The watchtower appears most memorably in Isaiah 21:8-10. A sentinel posted on a watchtower waits to for some unknown event. Finally, pairs of riders show up bringing the news, "Fallen, fallen is Babylon; and all the images of her gods lie shattered on the ground." The watchtower may be a literal tower, but it also means occupying a position of hopefulness and awareness. The sentinel in the watchtower could be any person who waits in expectation of the Kingdom of God, by adhering to righteousness and God's laws.
But the watchtower gets plenty of play outside the Bible. This passage from Isaiah inspired Bob Dylan's famous "All Along the Watchtower," which imagines the watchtower as an outlook searching for signs of life beyond a world of petty crimes and futility. And the Jehovah's Witnesses named their main magazine, The Watchtower, after this symbol, suggesting that reading the periodical is itself a way of watching and waiting for the end of time and the coming of the Kingdom of God.