I picked up a plastic model kit of a Japanese castle at the Britcon bring and buy. I haven't figured out exactly which one it is, but it appears to be made by Doyushu and may be between 1/500-1/350 scale so 3-6mm depending on which castle it is.
I haven't seen these before so thought I would mention it. The scale is on the small side but as scenery, they are a good addition,especially for only £5.