Puzzle #169: Fortress Sudoku

For his last two puzzles for the Daily League, Tom Collyer has been playing around with the same layout in his Extra Region and Clone Sudoku and his Very Odd Sudoku. This second puzzle can be solved under a second rule. After having solved it both ways, I tried solving it under a few other rules that might be presented that way. None of them worked. But I found that solving it under Fortress Sudoku rules lead to some interesting deductions by removing a few of the givens. So with that opening, I started playing around a bit what givens would make it unique. This is the result. I think it turned out nicely.

Rules of Sudoku

In this Sudoku digits in grey cells are larger than all orthogonally adjacent white cells.

Click to enlarge