The poem's imagery paints a picture of nature's soft persistence overwhelming even the grandest civilizations.
Looking out over a countryside landscape at the "end of evening," the speaker feels as if the quiet is so deep that it's visible: this is a "quiet-coloured" world, a "solitary" place of utter peace where the only sound is the gentle "tinkle" of sheep bells. Unbroken even by a single tree, grass sweeps out to the horizon in a thick, plush "carpet."
You wouldn't suspect it, the speaker says, but beneath this unbroken "verdure" (or lush greenery) lie the ruins of a great city. Once upon a time, a "domed and daring" palace loomed over a city bounded by marble walls—walls with gates so grand that soldiers could march through them "twelve abreast" without feeling the least bit cramped.
All the speaker's images of this lost city emphasize its grand scale and its richness. This was a place where even the chariots were clad in "gold, of course." Now, though, the only sign of the city is a "single little turret," riddled with "chinks" (or cracks) and overgrown with vines and weeds.
The contrast between the images of the city—which once held "a million fighters" and a mighty court—and the present countryside, where only sheep are there to watch as the speaker meets his beloved "girl with eager eyes and yellow hair," stresses how utterly gone the city is, and how strange it seems that this should be so. It also makes the soft, dim, "quiet-coloured" countryside feel like a relief, a soft landing after a lot of martial chaos. Perhaps it's no bad thing, the speaker's imagery hints, that the sheep and the grass reclaim all cities, in time; the gentle "ruins" are a better place for love than that long-ago city would have been.