I figure this is part of the lower levels of the city which would be darker due to the heights of the layers going up. The lower down you get the more non-winged creatures you'd meet, and the more the architecture would cater to them, whereas the higher levels would get more light, but also be less accesible as they'd be designed for people who fly a lot and don't have to think about how anyone else manages. (Look we never said the metaphors were gonna be subtle)