You will have to decide this for yourself, but there are a couple of very strong visual images that you should consider. First you think about what exactly visual imagery is -- these are the places where the poet uses such great diction that you imagination can completely see the place or thing that is described. The first visual image of the poem is of "the saddest city lane." That is all the information you get, but the word saddest sparks your imagination to consider how empty this lane must be -- especially empty of other people. You can also think about what else would make a lane look or feel sad: dark, dirty, worn down, etc. The isolation of the speaker is enhanced in the next section of the poem when he encounters a watchman, but makes no real contact with him, and when he hears the sounds of other people, but has no contact or connection with them either. The last great visual image of the poem is the "great luminary clock in the sky." This is a metaphor for the moon. The position of the moon in the sky serves to suggest the time of night, and it suggests something that seems close, but is actually very very far away and inaccessible. The visual images of the earth and sky complement one another and enhance the feeling isolation and desolation that come with the night.