Perhaps the most renounced and recognizable era of development in Half Life 2's early days, this is when the story and level design started to lock in place with the familiar Washington-style City 17, the Air Exchange, the wasteland's Depot, and many more revised or cut locations. Characters such as Barney, Dr. Kleiner, Alyx Vance, Captain Vance, Dr. Mossman, and even Breen were beginning to make a sturdier appearance in what we know of the somewhat more concise story.
The Worldcraft Mappack contains around 300 .vmf (the file format that represents editable Hammer levels) files. These files represent levels and areas from almost every location Valve prototyped and playtested during the six-year Half-Life 2 development cycle.
By analyzing the last-modified date of these .vmf files, we can observe that the Mappack contains levels that were made from 2000 to 2003. These levels not only provide a general, accurate timeline of locations and gameplay concepts that Valve continuously prototyped and discarded, they can also provide deep insight on how the Source engine itself evolved during this timespan.
In terms of the general state of these contents, the Mappack provides levels that range from being almost completed and fully functional, to being in a very unfinished state. Many of the maps are also affected by engine compatibility issues - as the Source engine was evolving, many features were being modified or even discarded at a very quick rate, leaving many old levels very outdated in the process.
This means that, if anyone wishes to get the levels to run in an available Half-Life game, some work is definitely required. The nature of this work mostly revolves around fixing outdated entities and brush work, so that the game can correctly interpret and run the level itself.