You're going to need two things: a good stone/blocks pattern and a castle layout. Draw your castle in sections, put each section on its own layer. Like one layer for the east wall, one for the west wall, one for the south wall, etc. Then each of the turrets goes on its own layer. You can use really basic shapes to lay out your castle - squares, rectangles, circles, etc. Put a layer style of pattern overlay (the stone/blocks pattern) on each of the layers and also a layer style of stroke (black). If you want crenelations and roofs those also go on their own layers. Lastly, put an empty layer at the bottom and fill it with some green or a grass pattern.