One of the true successes of BioShock's development was our ability to identify and react when the game was not shaping up to become what it needed to be. For example, the first vertical slice prototype we built was an non-navigable linear corridor shooter that looked like it took place in an abandoned box factory.
It didn't provide a compelling experience as either an RPG or a shooter. In response, we threw away that prototype and started again from scratch with the goal of building a single room that felt like the ruined underwater utopia we were trying to build.
First we did concept art passes. Once we got a concept that worked, we built it. Then we used it as a demo space. We used that single room (now Kashmir Restaurant in the first level of the game) as an artistic reference that guided us in creating an aesthetic unlike any other game on the market. (For more about the artistic style of BioShock, see the free art book download here.)