Each turn you place either an X or an O — anywhere. The Attack is trying to build a 5-symbol route to goal (any direction). The Defence is trying to park the bus and run the clock out with no route ever completed.
Unlike tic-tac-toe, you don't own a symbol — every turn, either player may place an X or an O, in any empty square.
The Attack wins the moment 5 matching symbols line up in a row, column, or diagonal anywhere on the board — a route to goal — and it doesn't matter which player placed them, or whether they're all X or all O.
The Defence wins by parking the bus for the full 90: filling the entire board without that route ever forming.
Pick which side you want to play. The computer plays a strong, deliberately imperfect version of the other side — beatable, but you'll have to work for it.