The Castle of Shikigami, sometimes called the Castle of Nightmares, is the titular location from the Castle of Shikigami/Shikigami no Shiro shoot 'em up videogame series. It serves as the main setting in the 3 games, though it's exact role varies with each game. Nevertheless, the castle is always shown as a source of supernatural power.
In the first game, the castle is used by the Septentrion as a dimensional portal to summon the demon Shinen into our world. In the second game, the castle is summoned by Shintaro Kuga, who wants to absorb it's powers to attain godhood. In the third and last game, the castle is shown as a sentient being, arising by itself in the country of Alcaland, where it creates various illusions shaped after people known by it's visitors.