In ancient times, and in many places up to today, thorn bush hedges were planted to serve as both fences and defensive walls. It was practically impossible for an enemy or a predatory wild animal to get through a thick one. The hedge being a familiar sight and concept to everyone of the day, the word is used in the Bible as a metaphor for protection against evil. Examples are Job 1:10 and Matthew 21:33.
The old Irish Christians took the concept of the circle from the Druids to represent the same idea. St. Patrick recorded some of his prayers, and a famous one asks that God draw a circle around him as he sleeps, keeping good within and evil without.