Sorry, but I can't see how you hoped to get away with this! (and I speak as someone else who didn't strictly speaking meet the challenge requirements myself.) Using the sun clearly isn't allowed, and to be honest I'm not that bowled over by the photo either - maybe if you had used sunlight in a creative way I might have been tempted to ignore you not meeting the challenge.
The sun and moon are not artificial light sources, not even in shadows and tunnels. Therefore, to me, this is a free study shot. Also, I find the edges a bit out of focus as well as the background. For all of these reasons, I scored you very low on this entry.
I believe the challenge called for an artificial light source...
With that said, there isn't much here of visual interest. There is a natural frame, but it doesn't work as well as you might have intended. The colors are flat, and the lines aren't working. Can't figure out if you were going for a focal point or a vanishing point here. While not completely out of focus, this image just isn't as sharp as some of the others in the challenge. Overall, this looks more like a snapshot rather than an artistic interpretation of something. Better luck next time....
You will have got plenty comments about the sun not being an artificial light-source ... so I won't mention it. Composition is OK. I wonder if different lighting woiuld have changed the feel.
Interesting picture but I think you miss read the challenge. The light source is supposed to be “artificial”. I'm sure you will get some comments on that.