Center-Dot Sudoku

These puzzles are based on the original 9x9 cell Sudoku puzzle but an additional constraint is placed on finding the solution. The center cell in each of the nine 3x3 blocks has a grey background. In addition to the usual rules of Sudoku, these cells must also contain the numbers 1 through 9. There are 30 Easy, 30 Medium, 40 Hard and 20 Extreme puzzles as well as solutions and How-to-Play instructions.