Normal sudoku rules apply. Grey squares are walls and green squares are cave. Some of the cells must be coloured grey so that all grey cells (the walls) are orthogonally connected to the edge of the grid and all other (green) cells form an orthogonally connected area (the cave). A clue 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 towards 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.