If you have an image manipulation program (GIMP, Photoshop, Paint.net, etc) all you need to do is posterize and downscale the image to make it look like Castle, there's no need for specific programs and, if you want to make it look nicer by dithering, most image manipulation programs have dithering built in as well. Alternatively, if you really wanna get that Castle look, you can change the image color mode to pallette or indexed (depending on the program you're using) and then just change the colors of the pallette to Castle's color pallette.