Download at the New Super Mario Bros. Hacking Domain .New Super Mario Castle is a minihack of New Super Mario Bros. by Arceus , and is the first finished hack of its kind. It is the third-released original New Super Mario Bros. hack, and the seventh finished hack overall.
Castle is comprised of two worlds, with each level using a Castle theme. This hack has a high difficulty curve and is made as a challenge of sorts to New Super Mario Bros. players. There is a sequel in the works titled New Super Mario Castle 2, which is being made as a hack of New Super Mario Bros. Wii . You can find some videos of 1-1 and 1-2, starting from this thread.
Arceus is currently working on a full hack titled Super Mario Bros.: The New Worlds.