The speaker of "The Prisoner of Chillon" is admirable in a lot of ways: he's courageous enough to face life in prison without batting an eye and he's willing to be martyred for his political beliefs. He's also a good older brother, and tries to set a good example for his two brothers in prison.
But nice, clear sentence structure isn't one of his virtues: he speaks in long, rambling sentences. Just look at the first stanza (lines 1-26). That's all one sentence! He uses a lot of dashes and semicolons, which make the poem seem much more conversational. The speaker is talking to us (or to himself) about everything that has happened to him since he was thrown in prison. It's almost as though he's trying to understand it himself by going over it in his mind. After all, the time that passed since his youngest brother died is mostly a blank to him – he stopped counting the days at that point. So the long, rambling sentences actually make some sense, given the speaker's state of mind. He's trying to piece together the long, blurry period of time since he was thrown in prison, and his syntax reflects that.