Just a glance at the different symbol clusters (see the "Symbolism, Imagery, Wordplay" section for more) should tell you that "Freedom and Confinement" is a major theme in this poem. There are lots of recurring images of birds and chains, flowing water and still water. If the symbolic weight of these images doesn't convince you, just look at the title: it's the "Prisoner of Chillon." But the poem isn't just about literal imprisonment and the desire for freedom; it's about the different kinds of confinement we face – mental and emotional, as well as physical.
The speaker is freed from his literal prison at the end of the poem, but "the whole earth" is still "a wider prison" for him (322-3).
Although the speaker believes that "the whole earth" will still be "a wider prison" for him after he is freed, it's possible that, over time, he'll be able to recover from the mental and emotional shock of his long imprisonment.