from the Acknowledgements:
RetroQuest started as a derivative work from Daniel Proctor's excellent GORE™ rules. This edition of the game uses a D20 instead of a D100, to keep the “simple and fast” spirit of RetroQuest; it is mostly based and inspired on Ray Turney's Fire and Sword©.
Released under the Free Art license.
RetroQuest D20 Edition is not a d20-compatible game, it uses d100 BRP-style, roll-under mechanics simplified to use a d20 (ie 5% increments).