I'm not sure you're still interested in critique regarding the original picture, and, furthermore; lots of people already pointed a lot of stuff out. Nonetheless, I'm just going to add that I think the picture needs more work concept wise. I realize that its a picture of a fantasy scenario, but it still feels like you've mixed too many different cultures into it without adapting to another one.
I think this is why, apart from the strictly technical errors, the image looks a bit odd. You've got the renaissance/classic statues, standing in front of what looks like a gothic or maybe victorian (unsure) metall fence, above which floats a summerian/egyptian pre classic age throne, and It just doesn't work.
Looking at the wanderers approaching the gate, I get the feeling that they are exploring an ancient place, now desolated, (partly also because of how worn down the statues are); but I just can't picture any civilization or culture ever having put together such a mish mash of different styles. I think that you should shave more of an intuitive grasp over the concept or background present before you get to actually painting, just so that the picture becomes coherent (not demanding realism though).
And finally, I think you would find it a lot easier to get the scale right if you studied a little historical architecture and bought yourself a reference book with historical buildings, because there are lots of things which distinguish really huge buildings from smaller ones, (different forms of supportive structure being one example).
Furthermore if you did this you wouldn't need to throw in arbitrary stuff like the chains which do not seem to fill any function, but rather details that tell the viewer something.