And gray walls moulder round, on which dull Time Feeds, like slow fire upon a hoary brand; And one keen pyramid with wedge sublime, Pavilioning the dust of him who plann'd This refuge for his memory, doth stand Like flame transform'd to marble; and beneath, A field is spread, on which a newer band Have pitch'd in Heaven's smile their camp of death, Welcoming him we lose with scarce extinguish'd breath.
Our speaker is still describing Rome here.
The walls of the ruins decay ("moulder"); personified Time feeds on them like it feeds on an old ("hoary") piece of wood.
But, there's a pyramid above where Keats is buried, and it stands solid. This pyramid holds the dust (ashes) of the person who had it made in his memory. It also looks like "flame transform'd to marble."
Remember the fire imagery earlier in the poem? Fire represents eternal life here, while this pyramid is eternal life turned into stone.
A new group ("band") of humans have been buried in the fields below. The speaker compares them, using a simile, to a group of people setting up camp. But, you know, it's the "camp of death." Morbid much? Still, this band of dead people welcome Keats to their camp. He's at home there.
This merry scene of death is another way for Shelley to comfort the mourners.