First off, I'm not entirely sure if this is supposed to be here or in city mapping. If it's in the wrong thread, feel free to move it. Thanks.

So this is a sketch for my next (attempt at a) project. It's a ruined fort, abandoned for who knows how many years, if not centuries. It was recently found and claimed by a group of adventurers. The lighter green is the forest and trees surrounding the ruin. The blue is, obviously, some sort of shallow stream that provides fresh water to the adventurers. The black and brown are the ruins and new, wooden structures for defense respectively. The dark green is a path that formed due to the coming and going of the people who now stay there.

The entire clearing can be crossed in a matter of minutes so the place is not big at all. With about twenty adventurers the entire ruin is full.

Click image for larger version. 

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I did have some questions though. I've got one tree tutorial but that's for large overland maps, here it would look more like ugly shrubs. So I'd like to ask you guys if you have any good ways to generate some close up trees. And while I'm at it a tutorial for ruin like stones/walls would be nice as well.

Oh right, I work in GIMP only but I can probably turn a PS tutorial into a GIMP one in my head.

Thanks in advance.