Nightmare Castle is an English fangame produced by Noyemi K in RPG Maker VX Ace. A re-imagining of their previous project Amiha's Nightmare Castle, the game uses an 8-color palette and minimalist audio to simulate an early PC-98 game with Yume Nikki-style gameplay.
The gameplay of Nightmare Castle is similar to most Yume Nikki fangames, taking place in a dream world containing 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 player's goal is to explore the dream world and collect/buy effects that will change the player character's appearance and/or give them special abilities. The player can wake up at any point by pressing the Q key, and save by interacting with the calender in the protagonist's bedroom.
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.