Hello there! Looks good overall, but of course random little things:
It looks like you are overdoing with the dithering on the tanks, you need to choose more values and then blend with only a little bit of dithering. Here is an old post about dithering you should glance at. The barrel in particular seems a little strange, like you alternate bright pixels in the shadow, and you're shading the intersection of the barrel and the turret. That part shouldn't have any real dark values because they're basically on the same level, and the line between them should be a lot thinner, otherwise it looks like they aren't connected.
I think you should also think about not worrying about pixel art at all, 1282 tiles are pretty big for pixel art, you might be better off with either not worrying about keeping it pixel-y or size it down to 642 frames if you want the pixel art look. You could mess with Posterizing the filtered stuff, which will limit the colors and make it look like pixel art.
As for the fortress, keep in mind that you'll have to exaggerate features quite a bit on some stuff to get the top view to look right. Plus shading does a lot to make it seem more 3D.
Edit: Hmmm, attaching images is broken right now...