I love the composition! Beautiful colors and I really like the contrast between the cold environment and the warm glow of the torches and lava. May I ask what sizes you usually work with for your concept art? And do you use a special set of brushes? I really like to get into concept art but am never satisfied with the look of my brushes.
But if you're still sort of getting the hang of it, I recommend sticking with the circle brush as much as you can. As for size of the images, the bigger the better. I rarely work below 3500 pixels for either length or width.
If you want to get into concept art, check out Feng Zhu's youtube channel here: [link]