Defender

Your spaceship is on the left side of the screen.

You can move it up and down with the up and down keys or a joystick and fire with the ctrl key or the fire button.

The alien spaceships come from the right side.

You will gain energy if you shoot them up, but if they reach the left side, you will lose it.

The energy level is displayed as a bar at the bottom of the screen.

No energy = Game over.