“The Barad-dûr”

"... wall upon wall, battlement upon battlement, black, immeasurably strong, mountain of iron, gate of steel, tower of adamant... Barad-dûr, Fortress of Sauron. All hope left him...."

Frodo's vision of the Barad-dûr on Amon Hen, the Hill of the Eye, when he's wearing the Ring and hiding from Boromir. It's difficult to depict that almost absurd sense of scale that this implies - the movies didn't bother to try: "just stick an eyeball on a searchlight, it'll do" – but this, with a winged Nazgûl, by John Howe, is excellent and implies an even greater scale than it shows.