This game requires 4-5 players, so you'll need to set up several groups going at once. Each group needs a defined play area (use cones or discs) of about 10 x 10 yards.
In the middle of the square, place an extra ball on top of a cone or disc. This is the "Castle". One player stays inside the circle guarding it, while the other players pass the ball to each other and towards the castle to try and knock it down. Use 3 or 4 players on the outside depending upon your age range and how many players you have. With four players a logical square sets up, whereas with 3 players you'll find they'll need to move around a bit more (better for older players).
For younger players, a coach may want to guard the castle "loosely" so players have a good chance of hitting it. For extra variety, especially if you find your guardian to be hovering too closely to the castle, feel free to use two castles spread out a little bit within the square.