I really don't know

- why arcade is not on the list
- space invaders would be invalid

Shooter
First-Person Shooter (Wolfenstein 3D, Doom, Unreal)
Third-Person Shooter (Gears of War, Splinter Cell, Resident Evil 4)
Scrolling Shooter (Raiden, Gradius, Tryian)
Run and Gun (Contra, Strider)
If a scrolling shooter is valid, why isn't space invaders clone valid? The only difference is that in space invaders the enemys scroll and in a Scrolling shooter the background scrolls... the effect is the same! The enemys approach the player.... or am i dizzled?