A tall tower built so that people standing at the top can see a long way and give warning of anything coming. You find them on castle walls, along old city walls, on the edges of prisons and army camps, at borders, and in forests where someone sits and looks for smoke. The whole point is height: from up there you see an enemy, a fire, or an escape long before anyone on the ground does. It is mostly a word about the past and about serious places — you would not call the observation deck of Taipei 101 a watchtower, because that is for tourists, not for warning. The word carries a slightly heavy feeling, of being guarded and being watched, which is why it turns up so often in stories about prisons and about war. Two things you may meet outside history: in video games, climbing a watchtower to reveal the map is a very familiar action; and 'The Watchtower' is the name of a well-known religious magazine, so a search for the word may bring that up first.