The base and capital of the column seem compressed or cut off: in real life their height can exceed the thickness of the column. Of course, adding something on top of the column (perhaps a statue?) would make it less strange. If you don't want a large capital, the column might be broken off at a certain height.
The water pool tiles have unsightly sharp corners at the bottom (very unlike the rock corners close by). Maybe these corners are a necessary evil of tiling, but consider improvements.
Below the water, rock has bizarre blue pixels. If they represent water filling a porous rock, shouldn't water replace the black of empty "cracks" instead of the grey of rock?
The plants are nice, but complex enough to be repetitive: use them less and/or introduce variations.
The rock and grass tiles are brilliant. The only thing that could be maybe improved is the appearance of 3 very regular rows per tile of almost equal-sized rocks, but I suspect a more random layout would look more obviously tiled.