After completing the game you would be missing one achievement. In order to see the second ending and to unlock the last achievement you need to follow up on the hint the game gives you about instructions placed in the StreamingAssets folder.

1. Check the StreamingAssets folder for the ToCC.txt file: it would explain the level format.
   
2. Edit 30.ini file in the LevelData folder, change any 1 to 3 (1 describes an empty tile, 3 describes the exit tile).
   This works, for example:
   
   0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
3. Re-play the game to the 30th level or check Cheats.txt at the root folder of the game (this file exists only if you have the DLC, but the method works anyway) to find out how to skip levels ( click on the level's name trice ).
   
4. Complete the trivial 31st level.

All credit goes to this guide.
I've decided to create a guide in English because I've encountered a review from the player who assumed the game is bugged.