That is pretty good already. I assume you have several layers?
The more the better usually.

The fortress only?

The battlements could probably be done easily using illustrator paths and create some sort of pattern you can repeat over long distances. . I'll have to check.
You can also use paths with the feather is PS and curve it to make the towers battlements.

They added a texture on the roofs. You can add texture using the layer options or you can place one on the web manually using a mask layer on top of the roof layer.