Explore the Portal maze. By looking at the outside scenery, you can determine how the rooms are physically located in space. Order these from bottom to top.
Each room contains an image on the floor. Each image clues a word. The intended words are:
Some of these images are ambiguous, but they are disambiguated by the realization that, in Portal, the cake is a lie.
Each word contains exactly one letter not in the word CAKE. Removing those letters, the remaining letters spell STOP SIGN, the answer to this puzzle.
This is yet another entry on the list of puzzles based on an idea that was originally proposed as a metapuzzle, but for one reason or another didn't make the cut when we were choosing metas.