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 form 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, grey walls act as “Region Sum lines”: Meaning that each wall (ie group of connected grey squares) must visit at least two boxes and the sum of the digits in each box must be equal.