More on Alarum Bell from Infoplease:

• Alarum Bell - Alarum Bell In feudal times a 'larum bell was rung in the castle in times of danger to summon ...
• William Shakespeare: Macbeth, Act V, Scene V - Hang out our banners on the outward walls; The cry is still 'They come:' our castle's strength Will laugh a siege to scorn: here let them lie Till fam
• William Shakespeare: Macbeth, Act II, Scene III - Here's a knocking indeed! If a man were porter of hell-gate, he should have old turning the key.
• Dictionary of Phrase and Fable: A - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "A"