in a rush, will explain my edit later, should still be pretty helpful though:

you had way too many colors in that wall, so much so that there were 2 stragglers i couldnt find that made my image 17 colors when i only "used" 15.  the other thing is clarity. one of the most valued things of pixelart is clarity, it is unique to pixel art because the artist has FULL control over every single element of the image. the work you have now is good, but reminiscent of photoshop art where things are aaed unnecisarly and messily. i also pushed the higlights on the cracks, maybe just personal preference. i learned the technique from art in the game " chaos engine". go look it up, the art is pretty much awesome.