Dave Bonta's short poem puts us inside the head of someone imprisoned for what they believe. The title, "Prisoner of conscience," points to a person held because of their principles, and the poem shows how lonely that position is. The speaker is "a faction of one," a political group reduced to a single member, and even that one member holds a belief in doubt. That is a striking twist. Instead of firm conviction, the cell holds uncertainty.
The castle image deepens this. A castle "with no king" is a strong, fortified place with no ruler inside it, so doubt becomes a structure with nobody in charge. The last lines, with a king who might "wanton away his time / in volumes of pain," suggest idle hours spent among accumulated suffering, or perhaps books of it. The mood is bleak, quiet and inward. The speaker seems less defiant than stuck, thinking in circles, with doubt as the only company and the only thing to defend.