Up until the 19th century, cities often stationed watchmen on high walls or in watchtowers to keep guard. Their job was to keep an eye out for the enemy, by watching out over the horizon from their vantage point. They were the ones who warned the people of any apparent dangers. If they saw any sign of an attack, they would alert the people with a trumpet blast, or ringing a bell. However, if they failed to properly warn the people, by falling asleep or just being unaware, then the city could be attacked without the citizens being prepared to...