Tubeway

Tubeway is a simple shooter game in which the player uses paddles to move a small white cross-hair around the top of a "tube" or wall while firing down at the computer-controlled opponents attempting to scale their way up it. The opponents, known as the "Tubeway Army", consist of triangular green homers (100 points) and triangular blue seekers (200 points), both of which can return fire. A special opponent called the germ occasionally emerges from a white box in the lower left corner of the screen. The goal of the game is to clear as many levels as possible before running out of lives. An extra life is granted every 20,000 points.

Source: Wikipedia, "Tubeway (video game)", available under the CC-BY-SA License.