The difficulty level of the Sudoku is variable. This depends mainly on the number of possible numbers in the boxes to fill. Some Sudoku involve a lot of boxes for which there is only one possibility indicated by line, column or zone containing this box. For more difficult, it may be necessary to return several times back in case there is a blockage. So it is better to have a Sudoku software, rather than having a pencil and an eraser to solve a Sudoku. The ideal is to have software that detects collisions and solves Sudoku. I recommend Sudofun a free software on WindowsPhone that has these functionalities. it solves all Sudoku and it will help you out in case the game would become a real headache. It happens that Sudoku offered by publishers are more complicated than expected and the grids require too many in-depth research.