Adult PC Game: Doujin Circle Gyu-Knights of Messiah IF ~ Captive Subspace Castle ~ Archive

This is the story of Alicia and Maya, the heroines of this game, being ***napped by Prince Ritz of the Kingdom of Windsor.
Celis, Rini and Riku go to rescue. It is located in the subspace of the Kingdom of Windsor.
To find the Windsor Kingdom in the subspace, Celis and his friends explore a "magic stone" called a gemstone.