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Classic + Fortress
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Yes. Notes can help track which values remain possible when several comparison constraints overlap.
Fortress Sudoku adds comparison rules between neighbouring cells. Marked cells must be larger than adjacent cells, while unmarked cells must be smaller than their neighbours.
Yes. Row, column and box logic still applies, but you also need to consider the local comparison relationships created by the fortress markings.
Start with marked cells that already have strong restrictions from the grid, then compare them with their orthogonal neighbours.
The markings indicate whether a cell should be larger or smaller than the cells directly next to it.