Position the cross-hair cursor slightly in front of the
incoming bombs and press either the left or right mouse buttons
(corrosponding the the left and right missile bases). The incoming
bomb will be destroyed if it collides with your outgoing missiles
explosion. An incoming bomb can also be destroyed by another incoming
bombs explosion, sometimes creating chain reactions.
You are limited in the number of missiles you have to defend your
cities. As the levels progress, you are given more missiles to defend
against a growing number of incoming bombs.