The second stanza begins by describing the speaker's prison. The dungeon has seven Gothic-looking pillars (a Gothic pillar, if you were wondering, is one that holds up Gothic-style arches. Rather than go into an art history lesson, we'll just refer you to the image of a Gothic pillar in the "Best of the Web" section. Go check it out if you're curious, and then come back. We'll wait.)
The seven pillars probably represent each of the seven members of the speaker's family (his father, his five brothers, and him).
The seven columns are massive ("massy") and "dim" – no wonder, dungeons aren't known for their bright lighting.
Even the single "sunbeam" that seems to have found its way into the dungeon looks "imprison'd" down there. The speaker even says that the "sunbeam" has gotten "lost" – light doesn't belong there.
The ray of light must have worked its way in through some crack or "cleft" in the wall.
The sunbeam "creep[s]" over the clammy, damp floor of the dungeon, and the speaker compares it to a "meteor lamp" in a "marsh" or swamp. Sounds pleasant!
Each of the seven pillars has a ring on it that holds a chain. The speaker says that the iron of the chains "canker[s]," or festers and plagues him.
He says that the "teeth" marks of the iron are still visible in his arms and legs – they won't wear off until he dies (i.e., until he is "done with this new day").
Even the "new day" is a source of pain for him since he hasn't seen a sunrise in so many years that he's lost count.
He stopped counting the days and years they'd spent in prison after his last brother died, and he alone survived.