Whereas sudoku has only one rule: each number must be unique in the row/column/grid. The towers game adds another rule: Consider the numbers in the grids as height of towers, the numbers on the edge of the grids represents how many towers you can see from there (you cannot see a lower tower behind a taller one). The addition of this rule make the game much more interesting, and harder. It takes me half an hour to crack a 7x7 game. Don't take my word for it, try it yourself.