The poem opens with the first-person speaker looking back on his past and remembering a time when his life felt like a deep, impenetrable sleep—a sleep in which he was oblivious to danger. His "spirit" was in a dreamworld.
In just these first two lines, the reader might already get the sense that something is going to go wrong with this metaphorical "slumber." The past tense suggests that, while the speaker might once have fearlessly slumbered, he's awake now. And the language the speaker uses feels a little ominous and ambivalent. His slumber didn't, say, guard or embrace his spirit, but "seal[ed]" it, closing it tightly away and perhaps even imprisoning it.
The word "seal" might also suggest another kind of seal: a wax stamp imprinted with an insignia or initials, used to close letters in the days before envelopes. In other words, this "slumber" might not just have closed the speaker's spirit up, but stamped it with its own identity, possessing it. This would mean that the speaker was so deep in his sleep that his very soul seemed to belong to the dream-world.
Similarly, the idea of having "no human fears" might sound like a relief, but it's also a little sinister. If the speaker had "no human fears," he was safe, sure, but also distanced from his own humanity.
And though he seemingly had no idea he was asleep, dreams always come to an end.
The speaker communicates all this complex feeling in a remarkably straightforward way. This poem uses the simple ABAB rhyme scheme and the steady, pulsing common meter of a ballad (a traditional form based on folk songs). This means that the lines alternate between iambic tetrameter (four iambs, or four da-DUMs) and iambic trimeter (three da-DUMs).
The speaker builds a lot of sophistication into this simple template. Take a look, for instance, at the evocative way he uses end-stopped lines here:
A slumber did my spirit seal;
I had no human fears:
Those heavy closing punctuation marks suggest big, thoughtful pauses. These pauses, in turn, give the poem a pensive quality, making it seem like the speaker is reflecting deeply on his past. That colon in line 2 also feels expectant: the speaker is about to tell readers something more about his long-ago "slumber."