Steel Spin (also called Steel Spine) is a boss from Darius II, which later returns in Darius Burst and Darius Burst Another Chronicle. One of the various Belser flagships, Steel Spin is a versatile battleship that, despite it's size, poses a considerable threat. Not only it is extremely fast, it is capable of shooting lasers and needles in all directions. It is also protected by a ring of orbs that orbit the ship, which it can control to spread around or to protect it closer. It is shaped after a porcupine fish.
Steel Spin later received an upgrade in the form of a heavy armored unit called Mirage Castle. Mirage Castle gives it a massive boost in defense and firepower, though is severely reduces it's speed and mobility. The armored unit is comprised of various independent sections, which can be ejected in case of severe damage. There is also a second armored unit named Phantom Castle, which used different weapons bur works in the same manner. Mirage Castle and Phantom Castle can pair together when their armored units are ejected, forming the Dual Spin, with the two Steel Spin units acting in tandem.