Adventure 550, also known as "Adventure 3", was written in 1979 by David Platt. It extends Crowther and Woods' original game from 15 treasures to 26, and from approximately 130 rooms to approximately 240. (These impressive numbers are somewhat diminished by the fact that Platt added three new mazes to the original's two; this accounts for 56 of the new rooms.) Platt continued to maintain and extend "Adventure 3" for several years after its initial release, adding new hints and clever messages.
Platt's game engine (the parser and world model) was written in Fortran, but the game-specific logic and messages were programmed in a special-purpose language called "A-code", which Platt designed himself. It was the world's first adventure game description language, and arguably paved the way for later game description systems such as Infocom's ZIL, the Adventure Game Toolkit, TADS, and Inform.
This particular version of Adventure was written by Arthur O'Dwyer in C, following a version of Platt's A-code from 1984. Certain spelling errors and bugs in the original have been fixed, but otherwise the intent is to be faithful to the behavior of Platt's game.