Much like Crowther and Woods, the imps were initially inspired more by a desire to test their hacker skills than a singular desire for wealth. Indeed, in a famous 1979 article published in the scientific journal IEEE Computer, the authors promised to send anyone a copy of the game who sent them a magnetic tape and return postage.

This article, written by Dave Lebling, Marc Blank, and Tim Anderson, describes the game as a “computerized fantasy simulation,” and uses terminology familiar to anyone who remembers Dungeons & Dragons: “In this type of game, the player interacts conversationally with an omniscient ‘Master of the Dungeon,’ who rules on each proposed action and relates the consequences.” However, like Colossal Cave, Zork is primarily a game about exploration, involving such activities as breaking into a supposedly abandoned house, rappelling down a steep cavern, and even floating across a river in an inflatable raft. Along the way, the player is continuously confronted with puzzles and even a few fights (such as a troll to be dispatched with the sword). Most famously, though, the player must at all times be wary of the grue—a mysterious beast which lurks in total darkness, always hungry for adventurers.

On the surface, Zork appears to have much in common with its progenitor, Colossal Cave, and IF scholar Dennis Jerz has gone so far as to say that “whereas Adventure began as a simulation of a real cave, Zork began as a simulation of Adventure.” However, Zork offered several key innovations to that game, including a much more sophisticated parser capable of handling commands like “KILL TROLL WITH SWORD” and “PUT COFFIN, SCEPTRE, AND GOLD INTO CASE,” whereas Colossal Cave was limited to commands of one or two words (“GET BOTTLE”, “PLUGH”).

Zork also offered a more sophisticated antagonist, the famous thief, who roams about the world independently of the character and eventually plays an important role in solving the game. Zork II also introduces a coherent plot to add some narrative coherence to the player’s treasure hunting. Overall, though, the game was praised for its humor and excellent writing.

“The parser was far ahead of other adventure games and the environment was far better simulated (due to the fact that it was written in an object-oriented way). In many ways, Infocom's games were far ahead of their time.” – Marc Blank

Where [journalists and historians] tend to go wrong is to overemphasize the importance of our parser. You can't blame them for that, because we pushed it hard as a ‘unique feature’ as well. What was at least as important was coming up with good writing, good stories, and rich environments. – Dave Lebling

“Zork introduced an actual villain, the thief, who opposed the player character during the initial exploration of the dungeon, who could be exploited to solve a puzzle, and who had to be confronted and defeated. This was a real character with the functions of a character as seen in literature, not the mere anthropomorphic obstacle that was seen in Adventure.”-- Nick Montfort

I should mention that the mainframe Zork was not broken into a trilogy, but rather existed as a single massive game. After the imps founded Infocom and decided to commercially release the game for personal computers, they were faced with stiff memory limitations (and a wide variety of incompatible platforms). To get around the problem, they broke the game up into three parts, though not without some modifications and additions. It’s also worthwhile to mention the brilliant design strategy they followed.

Rather than port the code to so many different platforms, Joel Berez and Marc Blank created a virtual platform called the “Z-Machine,” which was programmed using a LISP-like language called ZIL. Afterwards, all that was required to port the entire library to a new platform was to write (or have written) a “Z-Machine Interpreter,” or ZIP. Scott Cutler took on the task of creating the first ZIP, which was written for Tandy’s TRS-80 (aka the “Trash 80.”) Indeed, one of Infocom’s key assets as a text adventure publisher was the ease with which they could offer their games on a tremendous number of platforms; graphical games were much harder to port.

“It was so (relatively) easy to write an interpreter for a new computer, and then – voila! – the entire Infocom library was available on that machine. So it was cost effective even for machines like the NEC APC (with its huge 8” floppy disks) where a game might only sell two or three dozen copies.” – Steve Meretzky

“The fact that we supported multiple platforms actually had some negative impacts, especially as the newer machines began to have more memory and better graphics. We had to write to the lowest common denominator, or spend time on each game fitting it to the different platforms.” – Dave Lebling