with some editing this could work on a 1080p camera...

for maximum cinematics focus on the area where the rain falls in front of the castle so the castle is blurred but the rain is clear and maybe you could like put the camera on the ground for a low angle shot pointing up towards the castle if you're trying to make it looks big. the problem here is that you'd have to do some editing on the atmosphere to make it seem more rainy outside instead of a sunny day, or you could wait till it's overcast and then control the rain using the hose and then not have to worry about the editing of the sunny sky