Duotronic

Played on a 12x12 board, this is a fast and easy to understand game where opponents try to possess as many squares as possible on a shrinking playing area.
Players take turns placing one stone anywhere on the cells of the board. Once placed, a stone cannot be moved unless it is captured.
Capturing: there are two ways to do this, either by placing one of your stones between two of your opponent's stones (which are then captured), or by sandwiching one of your opponent's stones (which is then captured) between two of yours, or between one of your stones and the edge of the board or an obstruction in any direction.
Obstructions: when a stone is captured it is removed from the board and replaced with an obstruction (these can be represented by coins, buttons, bits of paper, mosaic tiles, etc.) that cannot be played on, effectively removing the cell from the board.
Winning: the player with the most stones on the board after every cell has been occupied or obstructed wins.