What does "watchful" mean?

Watchful: Full of watch; vigilant; attentive; careful to observe closely; observant; cautious; -- with of before the thing to be regulated or guarded; as, to be watchful of one's behavior; and with against before the thing to be avoided; as, to be watchful against the growth of vicious habits. "Many a watchful night." Shak. "Happy watchful shepherds." Milton. 'Twixt prayer and watchful love his heart dividing. Keble. -- Watch"ful*ly, adv. -- Watch"ful*ness, n.