The Collaborative International Dictionary of English v.0.48:

Watchful \Watch"ful\, a.
   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.
   [1913 Webster]

         'Twixt prayer and watchful love his heart dividing.
                                                  --Keble.
   [1913 Webster]

   Syn: Vigilant; attentive; cautious; observant; circumspect;
        wakeful; heedful.
        [1913 Webster] -- Watch"ful*ly, adv. --
        Watch"ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

watchfully
    adv 1: in a watchful manner [syn: vigilantly, watchfully]