Fill and Cross. Trick or Treat Free is a fun yet brainteasing game for Mac that provides you with a wide range of challenging levels. The app brings you plenty of nonograms (picture puzzles) which you need to solve before you run out of time and moves. It also comes with a detailed help guide and provides you with nice graphics and spooky background music.
Your goal is to mark the board's squares following the column and line indications and reveal the hidden binary image. The game is over if you don't comply with the puzzle's rules (filled and empty squares indications found on top and left sides of the screen) and uncover two empty squares during a mission.
The app lacks an undo button and it allows only two mistakes per level. These two things make this game even more challenging than it already is, especially when you're trying to solve a difficult puzzle. Personally, I don't find these aspects as disadvantages.
You can make in-app purchases if you wish to have access to additional lives or increase the amount of mistakes permitted per level.
To summarize, you should definitely try Fill and Cross. Trick or Treat Free if you're looking for an entertaining puzzle game. The app is filled with dozens of missions, automatically saves your game progress, and comes without a price tag.