I think that one of the most basic elements of message comes from the inspiration of the title. Murdoch speaks to what "the sandcastle" might be from her own life:
I can recall, as a child, seeing pictures in English children's books of boys and girls playing on the sand and making sandcastles – and I tried to play on my sand. But a Mediterranean beach is not a place for playing on. It is dirty and very dry. The tides never wash the sand or make it firm. When I tried to make a sandcastle, the sand would just run away between my fingers. It was too dry to hold together. And even as I poured sea water over it, the sun would dry it up at once.
The idea of seeking to construct something out of that which is "very dirty and dry" represents one of the basic messages of the novel. Bill Mor is immersed in a life that is "very dirty and dry." There is much in his world that creates a very lifeless setting. Despite the dreams and hopes that live in building a sandcastle, there is much that exists which is "very dirty and dry." The most beautiful of creations remain in the subjective and are unable to find much in way of externalization.
Such a statement about human futility becomes one of the basic messages of the novel. One of the stylistic means through which this emerges is through characterization. Bill wishes to create his own sandcastles through a life where much is "dry." There is not an overwhelming attachment between he and his children, while the relationship between he and his wife is a frayed one. The presence of freedom in building his own vision of a "sandcastle" comes in the form of Rain Carter. The opportunity that she provides him is one of a new world where he can "play in the sand." The characterizations offered are one way in which Murdoch is able to articulate the dynamic between our constructive and transformative powers of imagination and the tethered world in which we live that causes us to settle for what is as opposed to what can be. In the end, Bill accepts the reality of the condition that surrounds him, accepting the responsible and socially acceptable conditions of what is in embracing marriage and running for political office. Perhaps, in this light, new "sandcastles" can be built, but they will still be created out of that which is "dirty and dry." The prospect of finding new realms of sand, something that potentially existed with Rain, is absent. The stylistic element of characterization along with the expanded metaphor of the sandcastle are ways in which Murdoch is able to illuminate the novel's message.