i do like this a lot. I appreciate that you used rounded corner bricks for some of the corners just to make it look a little better.
The roof looks basically fine, but that main section of large green roof seems to be the only part that negates from the total image of the castle. As others had said, do stuff to the roof, but only that large segment.
About the "fence", I agree that it should be redone as a wall encompassing the edges of the raised island part, but it looks fine as a fence. Baileys running along the very edge with an 8 brick high wall and overhang would be a good addition to the castle