The term 'watchful guardian' can be summarized by the crossword solution 'ARGUS, ' which is a five-letter word. ARGUS is a figure from Greek mythology who was known for his hundred eyes, which allowed him to be an ever-vigilant guardian. He was appointed by the goddess Hera to guard the heifer Io, who had been transformed into a cow. ARGUS' watchful eye and vigilance has come to symbolize a guardian who is always alert and attentive.