Gordimer's story is a political commentary on the system of apartheid that governed South Africa at that time. The minority white population was kept legally separated from the majority black population. The whites had most of the resources and lived increasingly walled off from and in fear of the oppressed black population. Whites felt that by staying apart from the increasingly angry blacks they would keep themselves safe.
Gordimer's theme or message in this story is that walls won't keep us safe. (She universalizes this theme by not setting her tale in any particular place or time.) The last paragraph reinforces her theme by showing what happens when the family tops the wall around their house with razor wire. Their beloved young son, after hearing a fairy tale, pretends to be the prince in Sleeping Beauty. He tries to scale the wall of their home and gets tangled up in the razor wire and killed. In other words, by putting up walls to keep out the undesirable "other," we end up destroying ourselves. Walls and razor wire are not the answer.