The Lost City
From Halopedia, the Halo wiki
"The Lost City", also known as "Forerunner Condo" to Bungie and "Cuesta Verde" to Vic DeLeon, was a visual look-development level created during the development of Halo 3. It was cut during the development process, as the space had grown so immense that "it could be a game in itself".
The final iteration's exterior appears to have influenced the design of the final game's Cartographer: the forward half of the city, especially its spire, is virtually identical to the map room's exterior structure. A building identical to the third iteration was also featured in the Halo Legends episode Origins.
Development and design
Built by Bungie environment artist Paul Russel, the level was to be based around a massive Forerunner city and served as a visual look-dev test for the game's campaign. By the time of working on his third Halo game, Russel's ideas for Forerunner architecture were beginning to run thin, and the artist wished to create a city that "got past" modern notions of cities - instead of individual buildings, the entire city was to be made of one single building. The project aimed to create spaces that had cultural function and implied Forerunner daily life via the architecture, as well as indicating what Forerunner values were. More than just "cool shapes with mysterious purposes", the structure was to tell a story of the people who used it.
At first, Russel started exploring the idea by designing different types of skyscrapers based on Forerunner aesthetic, gradually adding detail until discarding the idea of a conventional, human-like city in favor of a more solid and defined arcology design. Russel then built several architectural components, stacking them together to form a single, massive structure which he then flipped on top of itself. However, this design was also discarded and replaced by a single building covered in an ornate, extremely detailed cantilever pattern. The final design incorporated a variation of the repetitive cantilever structure based around a massive arcology structure, featuring an expansive interior space with complex structural elements nestled within the superstructure.
In total, there were around ten versions of the city produced. Once the architectural blockout was made, it was passed on to Vic Deleon, who populated the area with vegetation, entropy and lighting and other detail, working for around three weeks on the project alongside a cut level called "Tsavo Township" before being moved to making maps for Halo 2 DLC. By this time, the fiction surrounding the structure that it was to be a housing complex for Forerunner scientists living on the ring - possibly the Librarian's team. The idea of this iteration was to design an abandoned living space with a courtyard, that had fallen into disrepair.
Dan Miller was one of the designers assigned to work on The Lost City level, and later relayed that the level was cut as the development team felt it would give too much away about the Forerunners and their mystery.
Vic Deleon's art pass
The near-final iterations by Vic DeLeon, running in the "Halo 2.5-3" engine, c.2005.