Connecting dots horizontally and/or vertically as a wall to form a single closed wall. There should be as many walls on the sides as the number in the cells.
0 : There can be no walls around.
The four sides of the cell with 1,2,3 numbers will have as many walls as the number. There is no wall restriction for the edges of the cell where there is no number, there may be no wall at all, or there may be a wall on at most 3 sides.