Copyright (c) 1998 Pierre Baudy

Squaryd is played on the following 7x7 square board:

Each player also starts with a number of simple and double pieces offboard.

SQUARE - A square pattern made by four friendly pieces of the same kind (see below for some examples)
TURN - On each turn, each player may drop any number or double pieces and then must drop one simple piece, always at empty cells.
If the opponent threatens to make a square in the next turn, the player can only stop him by playing a double piece at the remaining corner. 
GOAL - A player wins if he makes a square with simple pieces, or loses if he makes a square with double pieces.

Squares are not just formed by orthogonal lines, they can make any slope with the board edge.

An example

Black's turn. If he drops a single piece at cell [1] he threatens to make a square in the next move (d1d3f1f3). So, after that Green must reply by dropping a double piece at [2]. The problem is that doing that Green makes a double square (d1c3e4f2) and loses the game.

Check this page for more information.