Adventure Construction Set (ACS), published by Electronic Arts starting in 1984, was one of the first RPG creation tools. It was written by Stuart Smith and patterned after his earlier RPG's Alibaba and the Fourty Thieves and Return of Heracles. ACS was eventually released for the Apple II, C64, Amiga and PC, but unfortunetly never got ported to the Atari. A while back I started to play with the MSDOS version and though it would be an interesting project to reverse engineer the file format. This turned out to be a lot harder then I expected but the results can be found on this page.
ACS Viewer is a tool written in C# that allows you to view the game data from the C64 and MSDOS versions of ACS. Although I haven't figured out what every byte of the file format does, the view does work with all the out of the box games and all known user created games. ACS Viewer also provides a rudimentary play feature that allows you to walk around the map. I would like to expand this into a full player some day.

Windows Installer
Source Code on Github