Why do u need a human to guard you,when you have the fearsome seven-headed Naga as a sentry! The benevolent serpents in Hindu mythology are guardian of the patala(underworld)and protector of great treasure. The Naga-although a very old motif at Angkor -- gets primacy at angkor Wat because it is at Vishnu temple, and Ananta Sesha is one of the two (along with Garuda) vahanas of Vishnu. Ananta sesha (the one without an end) is the nagaraja (king of serpents) and protects Vishnu within its endless coils.
From @k-choudhary