In the fall of 1989, Brøderbund published game designer Jordan Mechner's seminal platformer Prince of Persia for the Apple II. Now, in 2017, a fan has released a level editor for that original Apple II version of the game: leapop.
Fellow devs may appreciate that the editor is licensed for modification or redistribution (under the terms of the GNU General Public License), so you can download the tool and poke around to see how it was done.
In a broader sense, it's just nice to see a game released in '89 still maintains a mod scene, almost thirty years after its release. Such enduring fan engagement is rare, though not unheard of; the original Doom modding community is still very much alive and kicking, for example, even though the game was released over 20 years ago.
For more insight into how the original Prince of Persia was made, have a look back at this classic postmortem of the game Mechner gave at GDC a few years ago.
Better late than never! https://t.co/45gTpIE3PZ— Jordan Mechner (@jmechner) April 4, 2017