as vigilant as Cerberus[

Just learned of this yesterday.

I googled Cerberus and found this: "in Greek mythology, a three-headed, dragon-tailed dog that guarded the entrance to the lower world, or Hades . The monster permitted all spirits to enter Hades, but would allow none to leave. Only a few heroes ever escaped Cerberus's guard; the great musician Orpheus charmed it with his lyre, and the Greek hero Hercules captured it bare-handed and brought it for a short time from the underworld to the regions above. In Roman mythology both the beautiful maiden Psyche and the Trojan prince Aeneas were able to pacify Cerberus with a honey cake and thus continue their journey through the underworld.

http://www.cs.dartmouth.edu/~tim/cerberus/