Nightmare Castle is a fangame produced by Noyemi K, a re-imagining of their previous project Amiha's Nightmare Castle (悪夢城のアミハ). The game uses an 8-color palette and minimalist audio to simulate an early PC-98 game with Yume Nikki-style gameplay. The game contains a variety of different dreamscapes for the player to explore as well as a collectable currency that can be be used to buy effects from a shopkeeper.
The gameplay of Nightmare Castle is similar to most Yume Nikki games, taking place in a dream world with the ability to wake up at anytime where the player must explore various locations, collecting effects that change Ruho's model and give her extra abilities.
Scattered across the worlds are treasures, a collectable currency that can be traded to the exploring shopkeeper Masha in order to gain effects. Nightmare Castle also features an element of randomness to it: upon entering a doorway, players will sometimes be teleported to a different part of the dream world.