Prefabricated

* See also Rapture Construction and Primary_Construction
* See also UnderWater_Construction
* See also Concrete_IS_Your_Friend and Rebar
Economy In Building Rapture AND Difficulties of Working in Water
Prefabrication : To fabricate or construct beforehand. To manufacture in standardized parts or sections ready for simpler/quick assembly or erection as buildings/structures.
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Pre-Fab :
When the many parts of Rapture were built, they could not simply be just 'thrown together'. With the major difficulties at those watery depths, Construction had to be done with great deliberation/effort.
Many extra limitations and pre-provisions for structure interconnections and utilities and access ( not just watertight, but watertight under 280 lb/sq of external pressure )
The MMORPG would have Rapture make use of much standard construction ( form cast pre-fab sections ) for the external and general internal volumes. Afterwards, the INSIDES can be more variously shaped with false walls and lesser non-structural/non-load-bearing construction ( utilities, partitioning, decorations, etc ... )
More customization of (inward facing) facades would be done ( when you cannot have the full in-the-round/stand-alone for proper Art Deco style architecture ). This reflects a fundamental aspect of "Art Deco", where a rather plain/simplified/abstracted functional structure then has added decorations 'glued' onto it. Hopefully, this can make up for the lack of style on most of the City's exteriors ( where little is seen in the darkness of the light absorbing water ).
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Building Rapture Bits - Prefab ("Rapture Wasn't Built in a Day, But ..."):
Rapture's tall buildings, even my thick concrete ones without some proposed thinner shell of Ryanium (or [[Burial_At_Sea_Illogic#aluminum|Aluminum]), would if sealed and filled with air, float. That would allow partial construction near the Ocean's surface, where it would be simpler/easier than doing constructing at a dark and difficult 600+ feet down. The structural shells could be pre-fabricated in sections at a shallow enough depth for conventional divers to work, and within reach of above-surface machinery/workers/infrastructure ( at less than 100 feet ). Much smaller sub-sections also could be pre-assembled out of the water on ships/barges, and on land, for additional convenience/efficiency/cost-effectiveness. 
The fabricated sections would then be slowly sunk ( pressurizing as they descended ), tied into their anchoring foundation on the seabed, and connected/fastened together to build those tall and sprawling structures we've seen. Once whole, secured and watertight, all water would be pumped out to allow the work on the interior spaces ( inner walls and cosmetic surfacings, utilities, etc ... ), which still would be a substantial amount of work, but doable via ordinary means, and by workers NOT wearing cumbersome diving suits.
This 'shallow construction' process would require fewer men to be in those complex/unwieldy/EXPENSIVE hardsuits( those being required below 150 ft depth ), and would still be easier than many of the automated deep-water construction methods also being used. The top of Mt_Ryan ( The 'Sea Mount' undersea volcano Rapture is built upon ) has a good sized area within 100 feet of the waters surface where much of this pre-building work could/would be done.
The building components and assemblies (small and large) also could be built elsewhere (including on land) -- that being MORE convenient, particularly for materials, AND where environmental factors are much friendlier, to then subsequently be transported to Rapture's location. Having fewer construction workers at the Rapture Project construction site meant better control of 'secrecy' and fewer unwilling/problematic 'out of work' immigrants to deal with in the future.
Ryan would have had some real deep-sea Oil Platforms projects to act as 'cover' for Rapture's construction process - Both as fake destinations for the 'sections', AND to use such platforms to do construction away from the prying eyes of 'the ant people'. These earlier real projects would have been used to perfect many of the techniques used to build Rapture. 
The pre-constructed components made on land would be moved to some unmentioned destination ( and eventually Rapture ). Ryan's trusted employees would obtusify/obscure the details ( the financing would be out of his own pocket and from the other Rapture investors, so no bothersome Stockholders to ask inconvenient questions ).
Reducing the cost/effort/time/complexity to construct many of Rapture's buildings would help in getting it done in the timespan we are told of, and to reduce the monetary cost - allowing Ryan's City to be near as big/grand as what we see.
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ALOT Of Prefab Done Back On Dry Land :
* Costs less - use of EXISTING efficient manufacturing up in the Surface World.
* Transportation - you would have to transport the materials anyway if you made stuff in Rapture.
* Construction of pre-built-up Components - far easier than doing alot of detail work IN WATER - manually or even if mechanized.
* Efficient/Effective Pre-Testing (culling out rejects) of the units (before transport) can be significantly cost saving.
* Much would still have to be be done at Rapture's Building Site - up above water or just immediately below ( above-water machinery is far easier to handle ).
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