I would speculate that it's a 3D model with NPR techniques applied to it. The perfect perspective suggests that the image was rendered at some point (it's possible that it was then hand-painted). The details in the foreground fading to the bluish haze in the background says that the distance from the viewpoint is being used as both fog depth (the coloring really says aerial perspective) and simplification amount. Each polygon is minimally shaded except for what appears to be some ambient occlusion. The color scheme reminds me of Gooch shading for some reason.

However, all of these techniques were in use by artists with pen and brush long before the advent of CG artwork so I'm just speculating.