The Fountain of Vaucluse", by the american painter Thomas Cole (1801-1848). The paintings is now in the Dallas Museum of Art. Even if some details don't match, I found it very inspirationnal for the famous Keep on the borderlands.
The Fountain of Vaucluse isa small village in the south of France, well known for the impressive source of the river Sorgue. The middle-age poet Petrarcus did lived there and composed some of his most famous writing in this place.
The castle istelf did belong to the Bishop of Cavaillon, in the place where an antique bishop, Saint Veranus, defeated a dragon.
More recently, archeolgist found thousands of Vth century golden pieces as a tresaure in the source. Definitively a D&D place...