Third-party Modifications
In its original Spectrum version, Jet Set Willy has a clear separation between the game engine and the data describing the rooms. The rooms themselves are stored in a straightforward format, with no compression. It is therefore relatively easy to create customised versions of the game.
The review of JSW in issue 4 of Your Spectrum included a section entitled 'JSW - A Hacker's Guide'; remarks in this section imply that the author had successfully deduced at least some of the data structures, since he was able to remove sections of wall in the Master Bedroom. The following year, issue 13 contained a program that added an extra room ("April Showers") to the game, and issue 15 described the data formats in detail.
Several third-party editing tools were published between 1984 and 1986, allowing players to design their own rooms and sprites. Since then, these and other programs have been used by fans to create many modified versions of JSW, ranging from relatively minor changes in a few rooms to completely new games. In recent years, a Windows-based JSW editor has been created.
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