River Keep Ruins - WiP

[TheDrinkie]

I've been working on this ruined river keep for a few days now, and I've reached a point where I'm no longer sure how to continue. This is the first map I've tried to create and I've only been using Photoshop for about a week, so my skills are pretty limited. I perused quite a number of the guides in the tutorial section, but all of them were based around the idea of creating regional or city maps. I wasn't able to find any for making maps on the encounter scale.

Regardless, I applied a lot of the filter and effect techniques from the guides and was able to get this far. I want to replace the gaps in the walls with rocky rubble, and also line parts of the island with rocks (sort of like a breakwater). However, I'm stumped as to the approach for generating good looking rubble that will match the style of this map. Any ideas for me?

Of course, the rest of the map is also open to suggestions and criticism as well.