Before the
FPS games in
computers and the
side scrolling shoot-em-ups in
video game consoles, video
arcades have vertical scrolling shoot-em-ups. Most of these games have a tilted monitor, and they have a
bird's eye view of a main character
shooting bad guys. They lasted a long time, with
Space Invaders,
Asteroids, and
Galaga in the 1970's to later games like
SNK's
Guerilla War to
Capcom's
Mercs in the 80's and 90's, and so on...
However, they don't translate well to home console systems in the 80's (NES and the Sega Master System) and the 16-bit consoles (SNES and the Sega Genesis) of the 90's because of the screens turned back to the horizontal rectangular orientation in TVs. That's where the side scrolling shoot-em-ups start to be popular in consoles.