Excellent question. It is very interesting that this, one of Wordsworth's most famous poems, is not actually based on the countryside or the Lake District of England, where he had been inspired to write so often, but on a big, polluted city! This sonnet clearly demonstrates that Wordsworth could also be moved by the solemnity and magnificence of a sleeping city and not just waterfalls and mountains. However, if you read the poem carefully you will note that it is London as viewed from a distance, conveniently ignoring the squalor, misery and poverty that other Romantic poets such as Blake captured in their writings.
The city of London clearly produces a similar sense of calm and tranquillity as nature does for the poet. However, note how even in this urban Romantic poem, nature is used to show the beauty of the sight Wordsworth is contemplating:
Never did sun more beautifully steep
In his first splendour, valley, rock, or hill;
Ne'er saw I, never felt, a calm so deep!
Although it is London that the poet is describing, he feels the need to compare it to natural beauty to help explain how profoundly amazing he truly finds what he is viewing. Likewise, just as his nature poetry views nature as a whole as an organism, the last line of the poem, "And all that mighty heart is lying still," equally views the city of London as one organism. Thus, although the focus of this poem is very different, Wordsworth seems to use similar strategies to explain the similar impact that the sight has on him as in his nature poetry. This view fits with his view of nature through of the affect on him and the descriptive strategies he uses.