Normal sudoku rules apply. Some of the cells must be coloured grey so that all grey cells (the “walls”) are orthogonally connected to the edge and all other (green) cells form an orthogonally connected area (the “cave”). Grey cells may not have 2x2 blocks. A number in the top left corner of a cell indicates the sum of the digits that are seen from this cell looking to the north, south, east, and west. The cell with the clue counts once in this sum. Walls obstruct the view. A cell with a clue must be part of the cave. Within the field of vision of a clue, digits may not repeat. In addition, any two orthogonally adjacent wall cells must have values that differ by at least 5. Isolated wall cells are not bound to any constraint.