You might like to consider how the city of London is presented and how it becomes a reflection of Dorian's somewhat divided character in the story. We appear to be presented with two areas of London. There is the socially acceptable area, in which Dorian dwells with his friends, Basil and Lord Henry, and then there are the parts of London where Dorian indulges in his illicit vices, such as opium dens, brothels and other places. However, these two halves of London seem to suggest the way that Dorian is able to present a face of innocence and purity to the world on the one hand, whilst at the same time engaging in ever more decadent acts that spur on his own corruption. Note how we are first introduced to this darker side of London after Dorian breaks up with Sybil Vane:
Where he went he hardly knew. He remembered wandering through dimly lit streets, past gaunt, black-shadowed archways and evil-looking houses. Women with hoarse voices and harsh laughter had called after him. Drunkards had reeled by cursing, and chattering to themselves like monstrous apes. He had seen grotesque children huddled upon doorsteps, and heard shrieks and oaths from gloomy courts.
It is highly significant that after this act of destroying Sybil Vane, that arguably sets him on the path of corruption, Dorian discovers the darker side of London. Note how the description in the above paragraph gives this side of London an evil, dark and oppressive feeling, with its "black-shadowed archways" and "grotesque children" and "gloomy courts." Clearly this is a Gothic setting that hints at the division within the character of Dorian himself. From this point onwards in the book his life is spent divided between the "dark side" of London and the respectable side.