Complex sentence: It is played on a wall and with only one's hand to hit the ball, which is usually a standard tennis but in Dulwich College a patball is used, this is a hollow rubber ball.
Split into simpler sentences: It is played on a wall and with only the players' hands to hit the ball, which is usually a standard tennis ball. In Dulwich College, however, a patball is used; this is a hollow rubber ball.