Nine Circles levels are levels inspired from the influential level Nine Circles by Zobros. They are known for their difficult and epileptic waves sequences, which constantly switch gravity, speed, size, and copies. The rest of the level before and after the wave consists of series of other forms (the difficulty may vary), but they generally use the Tokyo blocks and sawblades introduced in Blast Processing (official Level 17). Most Nine Circles remakes usually leave out or add in some sequences from the original. They are also given a theme color each. For instance, Nine Circles is red, Problematic is green, etc.
More information in this page: Nine Circles Levels.