Really interesting! It does need an explanation - took me a bit of clicking around to discover the rules (which I actually enjoyed because it turned out not to be too complicated! But some will give up early).

One aspect I liked about the larger puzzle sizes it that the topology becomes nontrivial: you can work out for sure that a cell has to be marked without knowing the exact path from the edge inwards, which you later narrow down.

Feature request: can interior cells be marked? Right now it's disallowed which helps rule discovery but requires the user to remember more.