The case notes
How fortress cells work
A shaded fortress cell is larger than each orthogonally adjacent unshaded cell. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Gray cell > edge neighbours
A gray cell marks a fortress. Its digit must be higher than every unshaded cell sharing an edge with it.
Compare four edge neighbours
Look up, down, left and right. The fortress must beat six, three, eight and one. Diagonal cells do not enter this comparison.
Greater than 8 → 9
Eight beside the fortress does most of the work. The shaded cell must be greater than eight, and nine is the only Sudoku digit that is.
Higher than each neighbour
Nine is higher than all four unshaded neighbours. Check the cells that share an edge with a fortress, not the diagonal ones.
Read the transcript
A gray cell marks a fortress. Its digit must be higher than every unshaded cell sharing an edge with it. Look up, down, left and right. The fortress must beat six, three, eight and one. Diagonal cells do not enter this comparison. Eight beside the fortress does most of the work. The shaded cell must be greater than eight, and nine is the only Sudoku digit that is. Nine is higher than all four unshaded neighbours. Check the cells that share an edge with a fortress, not the diagonal ones.
