I thought id play around with this in photoshop for a few minutes. Not the greatest pixel art program but manageable.
I removed heavy shadows and added more highlights to the water. It is translucent and doesnt absorb light, just reflect, so there wont be heavy shadows unless it is in a dark container. clear liquid just distorts the color of the objects or containers it occupies, makes the color a bit lighter, and has tons of highlights.
I did a curves pass on your blue tank to bring up the contrast and the detail you put in, which feels way too subtle right now. I'd also recommend looking at patterns for the tracks, and add those to the big rectangles on each side for some nice detail.
And lastly i did a lil pass on the fortress, mimicing the color palette of the middle tank. Some light shadows, thickened cannons, stripe pattern, and softened the thick line in the middle base so the outer wall's line would be the most dominant.
Quick stuff, hope you enjoy and find some further direction!
And if you continue redrawing the fort, go for asymmetry!
Edit: aaah boo it garbled the fort... What site are yall using to upload clean sharp photos? It wouldnt let me link from my google drive or deviantart and i resorted to a private album on facebook to get an image in here..
Edit: I created a tumblr. Hope that works. The fort is much darker on my screen at work..If you want, I can lighten the ground a bit.