Numeri follows Slitherlink rules: you need to leave all digits valid with a SINGLE closed loop line.

The only difference is that you can only switch all segments around a square. It looks harder but it is actually faster than regular slitherlink. You just need some training as you have to think in a different way.

You can practice simpler grids using the lite version. It is still quite challenging. Inexperienced users frequently take more than 10 minutes to solve the lite version.

Cheers,
Lucio