Wow! Wonderful job. This is the stuff that inspires me to get going and do some art. Two questions. 1. Did you sketch in photoshop, or scan a sketch you did on paper? (If in photoshop, how do you like to work, brushes etc). 2. Did you work in layers, with the buildings in a layer in front of the ship?
Yes I do these 100% in photoshop, but very often when I'm too tired to sit on computer I do some brain-storming in my sketchbook.
I usually don't like to start with pure white canvas, so I have medium grey background layer and then new layer where I do my sketch.
I use layers always when I add something new or test something. I try not to use too much layers and I merge them soon as possible. I have lot of brushes, but I like photoshop default brushes, like chalk and round brush. Some texture brushes are also good!