Killer cages
Killer on the home screen is still a 9×9 Sudoku. Every row, column, and box must contain 1 through 9 once. Cages are an extra rule drawn on top of that.
A cage is a group of cells with a small total in the corner. Two things are required inside that group:
- The digits add up to the printed total.
- No digit repeats inside the cage, even when the cells are not in the same box.
A cage does not replace the normal rules. A digit that is legal for the cage can still be illegal for the row. You have to satisfy both.
Small cages are lists, not guesses
Two cells totaling 4 cannot be 2 and 2, because a cage cannot repeat a digit. The only pair is 1 and 3. You do not yet know which cell is the 1.
Three cells totaling 6 have only one combination: 1, 2, and 3. Anything with a 4 or higher is already too big once the other two cells are at least 1 and 2.
Two cells totaling 17 must be 8 and 9. That is often the fastest place to start a Killer board, because those two digits are then banned from the rows, columns, and boxes those cells touch — except for the one cell in the cage that will hold each of them.
After you place a digit, check the cage total that remains. If a two-cell cage showing 10 already contains a 6, the other cell is 4, and it still has to fit the row, column, and box. If 4 is already in that box, the 6 was the wrong cell of the pair.
How to play lists the same Notes, Undo, and Hint buttons. They behave the same way on a Killer board.