I have two points!
1 - Can me redirect to a tut explaining how to use terragen & blender together? Up to now I only cast a terragen render as a background to my pics, which is not as awesome as your work
2 - I’m from a culture which believes hell to be deep underground, so I expected the gates to be at the bottom of a mountain, not at the top.
Actually I was caressing the idea of making such an image, but I was thinking of rendering a scene from a poem written by a poet from my city around 1300 and describing a voyage to hell, purgatory and paradise. Poet who is actually the father of the Italian language and one of the greatest of Italy ever. You can found his visage on Italy’s 2 euro coins.
His description of the gates of hell are very inspiring, you might want to check the poem;)