Well done!
Thanks for asking about the game logic. The reason why you can only shift dimension at some locations is that it makes sure that you don't shift dimension and end up inside a wall. This is achieved by assigning random numbers (1,2, or 3) to each of the grid cells in the labyrinth matrix, where 1) is the white dimension, 2) is the black dimension, and 3) is a dimension shift location. Essentially dimension shifting is just like a taking a stair to another floor :)