Ninja Senki DX revels in its own simplicity, distilling the basics of 2D platformers into a single, beginner-friendly crash course in the genre. It's not the fanciest or the most difficult, but every aspect of it is carefully calculated to provide a satisfying experience for players of all levels of experience.
The Old Hunters is a bit on the short side and doesn't stray as far from the formula as it's proven it can, but what there is of it adds significantly to the depth of Bloodborne as a whole; Definitely recommended, especially for stalwart fans hungry for more, but nothing unmissable on its own.
The Deadly Tower of Monsters is an honest effort to create a good game based around an intentionally bad fake movie that ends up partially successful. It brings some interesting twists to a repetitive formula, but the sparse use of them coupled with its lack of length leads you to believe something important was left on the cutting room floor.
Amplitude is a labor of love on its creator's part. What it has to offer is a testament to Harmonix's mastery of the genre: Its simple yet deeply challenging gameplay and psychedelic graphics have been honed to near-perfection, and each distinct part of every song carries its own, faithful patterns and challenges. The critical flaw is that there's simply not enough content to go around.