My Humble Pentago Strategy Guide: Three Square
This is a good strategy that I like to use. When using this strategy there are many opportunities to deceive your opponent to what your goals are and if you are stopped by your opponent in trying this strategy it leaves you well developed for other opportunities and strategies.
I'll refer to the 3x3 blocks that move as quads or blocks and then describe where in the quad I placed a marble i.e. Lower left quad, middle left hole. (This hole is empty in the above picture.)
Let us play Black. White goes first and places in the center square of the top left quad. To be safe we block the center of the opposite corner. Twisting at this point doesn't matter
White places in the center of another quad and we place in the top middle of the lower right quad. Black doesn't twist the lower right quad.
White places just below blacks line of two marbles in the lower right quad because it is not a good idea to let your opponent get a block with three in a row. White twists this block clockwise. It would have however been better for white to block black by placing a marble in the middle of the lower left quad. It is just as effective of a block but leaves white with more opportunities.
Black places again in the lower right, starting to set up the three square attack. He twists the lower right quad counter clockwise. Notice that white must again defend against black getting three in a row on a single block.
White blocks by placing in the lower left center. White twists the lower right clockwise. Black places in the upper left now. He has a choice of four good places and picks the middle row left. Black twists the upper right.
White places in the lower left quad in the upper right corner of that block. White then twists the upper left block clockwise. Black places in the upper left quad in the middle right hole. Black twists the lower left block clockwise.
White, without question, knows black is trying for the three square win, but there is not much white can do. White tries to complete the diagonal win but there are now four holes white has to block black from but only one turn. (the four holes are the four corner holes of the upper right quad.
White places it's marble in the lower left quad, in the upper left corner. White then twists counter clock wise. White has three in a row diagonal and the center of the opposite corner, which in most cases is a good thing, but here it loses white the game.
Black places in the upper right quad in the lower left corner. Black twists the upper left block counter clockwise and wins.
Whites mistake was to mot place in the upper right quad in the lower left hole. White would probably have won the game if it had done so. Blacks mistake was to let white gain opposing centers. Black should have taken the it's time in development of its marbles and have not risked white getting a diagonal later on in the game. That is the main advantage of the three square attack, you can take you time to block your opponent and not be blocked yourself because there are so many ways to set up. You just need two marbles next to each other in outer diagonals of opposite squares.