Originally, Kurosawa was planning on building merely a facade castle for the film, but this proved to be an impractical step, prompting the building of full-on castle sections to use in shooting. These were built with the help of United States Marines who were based in the area.
Takeshi Katô (Guard killed by Washizu) was worried about the thrust of Toshirô Mifune's sword, so he placed a block of wood in his arm pit. Unfortunately Mifune's thrust split the block and wounded Katô. He carries a scar to this day.
The famous arrow scene near the end was in fact done with real arrows. That is, the arrows hitting the wooden planks were not done with special effects, but rather choreographed with archers. Mifune waves his arms to brush away the arrows sticking from the planks, indicating to them that he wanted to go in that particular direction. The real arrows were included to get Mifune's facial expressions of real-life fear, which is exceptionally hard to imitate. Of course, the arrows that hit the Mifune character were bamboo fakes.
Three great Shigenki theater actors had begged Kurosawa to play in the film. The director obliged and Isao Kimura, Seiji Miyaguchi and Nobuo Nakamura appear as samurai ghosts in the second scene featuring the witch.