The speaker announces a decision to run away to a sand castle, a place that sounds safe but is about as unreliable as shelters get. Sand castles wash away, and the poem seems well aware of it. Even inside this refuge, the speaker admits there are two things still to fear: time and anger. Time is what erodes the castle, and anger is perhaps what drove the speaker there in the first place.
The tone is dry and a little self-mocking. Promising to be "bland / as a discourse on ships" sounds like someone trying to make themselves dull and harmless, to feel nothing much at all. The last lines offer a warning that reads almost like a proverb: it is dangerous to "dally / with a little doubt." The poem suggests that even small doubts can undo a hiding place. The mood is wary and quietly anxious, wrapped in a calm, deadpan voice that doesn't quite convince.