Nice Sudoku-like number puzzle game - Towers

I found a game on iPhone App Store: Towers
http://itunes.apple.com/cn/app/towers-g ... 16862?mt=8

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.

You can search for the game in App store by "sudoku towers". It's free.