Near the end of the poem, the speaker urges mourners to visit Rome, where Keats is buried. It serves as a place for both contemplation and peace, and these finals stanzas contain quite a bit of Roman imagery. Shelley sees this as a place of eternal beauty and inspiration, and so he places these images near the end of the poem, where he finally reaches a place of peace.
He first tells mourners to go to Rome in stanza XLVIII. There, they'll find monuments that remind them of the eternal nature of life. That might comfort the mourners and remind them that Keats isn't totally lost. Stanza XLIX also urges mourners to visit the city. It's "at once the Paradise / the grave, the city, and the wilderness" he says (433-434). The city has a little bit of everything, including ruins of the once great empire.
These ruins lead to the spot where Keats is buried, in a field just beneath a pyramid. The imagery of this beautiful field juxtaposed with the ruins of the older world is also meant to remind mourners that life goes on, despite death.
As he says in stanza LI and LII, being near Keats' grave reminds him that there's nothing to fear about death.