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This article refers to Portal during its development stages from mid-2005 to its release on 10th October 2007.
A Steam user with connections to Valve acquired a development archive of Portal dated December 2006 back in early 2015 that contained materials, models, particles, and sounds that go back to 2005. The intention was to make a total conversion mod for Portal that allowed players to dynamically switch between development eras known as Portal Project-Beta.
In June 2016, the assets were leaked to Facepunch which resulted in the removal of Project-Beta.
A finalized version of Portal Project-Beta was released in May 2017, though the mod does not contain any new content and further development has ceased.
The actual puzzles themselves were finalized early on during development. The main differences between build iterations are the art styles themselves.
The 2005 iteration of Portal largely took design cues from Half-Life 2, especially Nova Prospekt. Most notably, instead of being guided through the facility by GLaDOS, the player was to navigate the facility alone and eventually escape. With the aforementioned omission of GLaDOS as a final boss, during the escape sequence the player would have to manually escape the facility. In the final escape map, the player would have to search for car batteries to unlock a locked door while avoiding hazards such as turrets and rooms on fire. The escape map is the only map available for download from this era and any other maps released are remakes based off of this map.
In early 2006, the transition was made from the Nova Prospekt design to a cleaner, more sci-fi look. The doors instead of being a brush based entity took on a new Star-Trek-like design. Some antiquated assets such as the analog clock and Combine elevator remained in this era however. The flame Portals as well as a newer, sleeker portal gun were also developed during this era. The indicator lights were also static orange lines instead of dotted lines.
Interestingly, in the Portal trailer as well as all subsequent media releases up until Portal's demonstration at GC 2006, most world props such as the exit doors and buttons remained untextured.
The Portal demonstration at X06 in Barcelona at the end of September 2006 introduced spiral portals, dotted indicator lights as well as a newly designed Testchamber 18, which arguably saw the most changes in level design over the course of development.Portal in late 2006 (and essentially all development eras before 2005) was largely similar to the final product, with the exception of slight modification of map layout, textures, lighting, and a differently animated portal gun as well as a different GLaDOS battle.
Instead of using a rocket turret to blow cores off of GLaDOS, the player was to chase a fleeing GLaDOS, using rocket turrets to damage her whilst avoiding turret fire. One of the reasons this boss battle was scrapped was because players felt sorry for attacking an essentially defenseless GLaDOS.
Notably, the room where GLaDOS was originally to be confronted was reused as the last turret battle before GLaDOS in the final game.
Portal in early 2007 maintains almost the same design as late 2006 with the exception of some small level changes, most notably a redesigned Test Chamber 18 which can be seen in the Game Informer sneak peek.
At GDC 2007, the retail Portals replaced the older spiral portals. A newer, differently animated portal gun model with the player's hand on the barrel can also be seen. Additionally the transition is made from a male character to Chell.