https://vitaliipolulikh.itch.io/dot-siege - free in the browser, no account, no download.

Dot Siege is a turn-based territory game built on the pen-and-paper game a lot of people played in the margins of a school notebook - Dots, or Kropki. You place one dot per turn on a free crossing. Your dots join up horizontally, vertically and diagonally, and the moment a chain of yours closes around enemy dots, you take them. The one deliberate change from the paper game is the one in the title. On paper the edge of the sheet is nothing - a dot on the border can never be surrounded, because there is no wall there to use. Here the board edge counts as one side of a siege, so border dots die like any other and the corners are the sharpest ground on the map.

The other rule worth knowing before you start: your score is the enemy dots you are holding right now, not the ones you have ever taken. Surround someone else's siege and the dots they trapped come back to life - and their score goes down. Scores fall in this game, and it surprises everybody the first time.