You could draw up your castle in SketchUp, switch the view to Isometric, rotate to fit properly, then export it as a jpg, and then in PS erase out the background leaving the castle. SketchUp is a free program like a cheap version of 3DS Max. Otherwise, yeah, you'll need to draw it by hand in a raster app or maybe have someone else do it and you guide them with what you want it to look like.