How does the cat symbolize and support the theme of "Once Upon a Time"?

The cat becomes a symbol and supports the thematic idea that fear as the real enemy of man. Once the husband and his wife install on their walls the jagged wire with serrated blades [referred to as Concertina wire and used at the top of prison walls and fences], they have imprisoned themselves in their own home. Therefore, just as the cat "kept to the garden never risking a try at breaching security," so, too, do they live.
 
In his poem entitled "Mending Wall" in which a man and his neighbor meet to repair a wall each year, the speaker of the poem remarks,

Before I built a wall I'd ask to know
What I was walling in or walling out....

Author Nadine Gordimer asks her readers this same question. She seems to agree with psychologists who perceive illusory fear as the greatest enemy because such anxiety feeds upon itself and generates what is often called a "whirlpool of negativity." This negativity, then, becomes impossible to escape. Like the fearful cat who has stopped climbing the wall and only stays in the garden, the family has imprisoned themselves behind walls and fences in their efforts to remain safe.
 
One night after being read the story of Sleeping Beauty and the Prince, the boy pretends that he, too, is a prince who must rescue his Beauty. He then takes a ladder so that he can reach her and kiss the sleeping lady back to life. When the boy reaches the wire on the wall, he tries to free himself from this tunnel of wire. Sadly, he becomes even more entangled and cut until he is but "a bleeding mass."