From this distance, it could be a pleasure ship, an apartment complex, or the first stately vessel in a military deployment. But the fortress is none of these things. It is the place where this gas giant's greatest secrets are kept, hidden in the clouds of the upper atmosphere. It's rare to glimpse it at all - you're lucky you were here to see it.

Created by William Smith, this piece of concept art is just one of his many beautiful images of the airships, blimps, and orbs that hover hugely in the skies of alien worlds. Smith's work ably conveys a sense of space and enormity - you can feel the vastness of the objects he's created in these incredible illustrations.

William Smith is a Texas-based concept artist, and you can see more of his work on his blog.