Here's part of a sample advanced game to illustrate some tactics.

After three moves 'X' is threatening to make a square worth 16 points by playing at B7:

Instead of blocking the square by playing at B7, 'O' chooses to play at D2, threatening to make a 49-point square by playing at F8:

'X' blocks this square by playing at F8:

But 'O' can still make a 25-point square by playing at G1:

'O' still has the option of making another by playing at A3 later.