The ruins are well done and look impressive and the water feels real, but the sky is a bit off. But I know that 3D is hard and I think this is a good experiment.
Very interesting composition. The unusual texture of the ruins is distinctive. I like the water with the algae a lot though I can't honestly say I'd want this picture on my desktop - too bleak and desolate - but this isn't necessarily a criticism of the image per se. It's a shame that the darkness of the ruins obscures the details somewhat. At first, it looked just lump-like to me even though I was looking at it full size. So, if I were to suggest something here I guess it would be to address that issue if you can so it is more clearly a picture of ruins and not of a lump. I think that would add a good deal of interest to the image.
I have to say I agree with margali about that lump thing. When I first saw it without looking the title I though it was a some sort of mountain...
But the water looks great! Better than I've seen people doing with that Terragen program... Keep experimenting! =)