I like the general color scheme and the epic nature of this angle. I'm confused by the city itself. On the right hand side we can see that there are buildings back there, suggesting that there's a city. On the left side it looks like this majestic structure has nothing behind it. Even if its only function is to be a wall, I'd add some subtle shapes behind it, otherwise it looks like that glowing white is indicating that there's nothing there. I also think the subtle rock arch in the background is cool. I'd almost want to emphasize it more, but maybe have it round out a bit more so that it acts as a composition border of sorts, keeping us in the frame.
That's a pretty awesome city gate, and there's some good atmosphere going on. Is there a way you could suggest more city directly behind the gate without make it too distracting? Right now you have some cool lights and clouds, but maybe those could be taller buildings that populate the city more.
I like the color scheme and the general feeling of it :) I think it could really jump off the page if you added a little value contrast to it, and it could really help emphasize the focal point!