In "Composed Upon Westminster Bridge," the poet is crossing Westminster Bridge into London during the very early morning. Wordsworth loved nature, but finds the beautiful, clear scene of the London skyline spread before him while he crosses the bridge as quiet and lovely as anything to be found in nature. The city is serene in the early morning, not yet awoken. He marvels over how still it is.
More specifically, in the first three lines Wordsworth says the city is so beautiful ("fair") that you would have to be a very callous person ("dull ... of soul") not to respond to its beauty. In the next two lines, he says the city wears its "silent" beauty like a garment laid on it. Its ships, domes and towers stand out "bright and glittering" in the clear air.
The next two lines note the sunlight falling as beautifully on the city as it would on any part of nature, be it rock, valley or hill. The poet shows his rising excitement over the stillness of the city in the last few lines. In line 11, he says he never saw or felt "a calm so deep!" and in lines 13 and 14, he again uses exclamation points as he notes the city's (he calls the city "that mighty heart") calm:
Dear God! the very houses seem asleep;
And all that mighty heart is lying still!
Given how busy and full of activity this major commercial city and center of empire usually is, it's no wonder Wordsworth is surprised at the calm. If, however, you have ever been in a city or town in the early morning on a clear, sunny day, before the bustle and activity has begun, you know what Wordsworth is talking about. What he most marvels at is that this sort of serenity can exist in an urban area, not just in nature.