Paint an acrylic splatter and sponge texture to start making the castle stones. Scan it to make a Photoshop file. In this particular job the horizon line is tilted hence the diagonal format for the stones. The cylinder in the upper right is part of a previously rendered castle tower.
In Illustrator create rectangles of different widths with the same stroke thickness. Stack these rectangles to make the enough stones for your wall. Select All then Copy and Paste into the wall texture photoshop file as pixels.
Working The Edges
The Illustrator paths are too smooth, so to add a little roughness click on the grid layer and select Filter>Brushstrokes>Spatter and adjust the sliders to taste.
Adding an Alpha Channel
Now select all and copy the stone grid. Click on the Channels palette and in the options choose and Make a Channel. Then paste the grid into it.
A Little Smoothing
Select All in the new Alpha Channel and go to Select>Inverse so that the grout between the stones is white. Since the Spatter filter may give you ragged edges that are too sharp, go to Filter>Blur>Gaussian blur and soften those edges a little.
Loading the Alpha Channel
Click on the layers palette and choose the Layer with the acrylic texture. Go to the Select Memu>load selection and choose the Alpha Channel that has the grid on it.
Once the grout alpha channel is loaded you'll see the marching ants selection edges (you can hide these by pressing Command H in a Mac). Now go to Image>Adjust>Levels and slide the middle triangle in the Input Levels to the left until the grout looks light enough for you.
Once the grout is lightened go to Select Menu>Inverse, to choose the area that will be the stones. Press Command J to put an exact copy on a layer directly above. Choose the layer with just the stones and click on the Layer Styles icon on the bottom left of the Layers palette. Choose Bevel and Emboss. Adjust the thickness, light direction, and color of the bevel to taste. Now click on Drop shadow on the left menu and choose the density, distance and color for your shadow.
Chips and Scratches
Hide the view on all layers except the Stones layer and the Grout layer. In the Layers Palette choose Merge Visible, Using the Dodge and Burn tools and various sized brushes add some distressing to the rock.
For more realism you'll add a layer of drips above the Stones layer. Use various brushes to create streaks and gunk in black or other colors. You can also use the smear tool enhance the dripping feel.
Change the Blending mode in the Layers palette to Multiply.
Adjust and Finish
Play around with the Opacity of the Drips layer to your liking and Voila! your done.