Look closely: each tile has 2 numbers which always add up to 9.
There are a ton of levels. The 2x2 grids are obviously quite simple. It starts to get interesting at the 3x3 levels. If you like those, you can also do 3x4, 3x5, 3x6, 3x7, 3x8 and 3x9.
I did the section of 4x4 which is more interesting. Of course the grid sizes get really big. The largest is 9x9.
Tilemetry is definitely an interesting math puzzle. This would be a perfect game for elementary school math whizzes. But you can still have fun with it as an non math whiz adult.