Gels that change the color of the light emanating from the cube walls help suggest different rooms, and careful continuity checks ensured it would all cut together. The engraved numbers at the entrance of each cell were costly, and they had to cut a few corners in the end. And that isn't the only place where budget forced compromises. Director Vincenzo Natali originally wanted to shoot the film entirely in sequence, but had to relent when the gel changes took longer than expected.
Digital effects complete the picture by adding in traps and showing the space outside of the cube.
The movie wouldn't have been completed with its scant budget had C.O.R.E. Digital Pictures not donated all of the digital effects work. In the original storyline, all of the characters were criminals and the cube was their jail. In that draft, all of the parts were male. The fact that there are six primary characters is no coincidence. The writers intentionally did this to reflect the number of sides of a cube.