eishiya has already told you the most important stuff, but I'll say a thing or two about the details/textures (all IMHO, of course):
-The green stuff doesn't look like it's growing on the surface of the tower, it looks like someone had a magical accident inside that resulted in a bunch of Green Slime™ getting thrown everywhere. If it's supposed to be moss, it should cling to the stones in a thin layer. If it's vines, you need more vertical strands or something. The current look might serve if the tower is located in a swamp or jungle, though.
-The wedge-shaped chunk missing from the upper parapet looks like someone sliced it off with a laser. Either follow the lines of the stone blocks, or give it crumbly/jagged edges.
-There's something not quite right about that chunk of rock sitting on top of the tower. It looks like it should fall over or something. Probably a perspective/shadow issue.
The door works fairly well if you ignore the structural curvature issue. The deep chink in the wall midway up, and most of the cracks in the stonework in the lower part of the tower in general, do make it look like the stonework was damaged and left without maintenance.
