Fighter Mode is an alternate mode in Castlevania: Circle of the Moon. It is unlocked by completing the game in Magician Mode and can be played by starting a new game and using the name GRADIUS (a reference to another series made by Konami.
Fighter Mode does not allow the use of DSS cards, making MP and the Intelligence stat worthless, but increases Nathan's whip and subweapon strength by x2 and effective vitality by x4. It is by far the easiest mode to finish in Circle of the Moon, even with the loss of many great advantages that the DSS provides(life regen, among other benefits).
Catacomb - Abyss Stairway - Audience Room - Triumph Hallway - Outer Wall - Machine Tower - Eternal Corridor - Chapel Tower - Underground Gallery - Underground Warehouse - Underground Waterway - Observation Tower - Ceremonial Room - Battle Arena