Another interesting game that I just found recently. Guardian Force is a Vert/Horz/Diag Shooter (that's right, Diagonal) with a rotating 2nd gun and some big, colorful graphics.
You control the action with a joystick and three buttons. Your middle button shoots both of your weapons. You have a forward aiming gun and a rotating special weapon. The outer two buttons rotate the 2nd weapon clockwise or counter-clockwise. Stop pushing these buttons and the 2nd gun stays at whatever angle it is until you move it again. So you can hang in corners or on the sides and still blast things coming your way - even if your front-mounted gun isn't doing much. As the levels go on, the screen starts scrolling diagonally (unprecedented!) on its way to eventually scrolling horizontally. In each case, your main gun always points in the direction of the scroll. So, when scrolling diagonally, your fixed gun is pointed up and to the right (diag) - and that's actually how I realized that the screen was scrolling that way. Very cool.
There are many levels of power - at least 10 that I have seen. And they power-up your weapons mightily. You increase your power/level by collecting the little pink-geo gems floating around. There is a level indicator at the bottom of the screen. You can also change your weapon type and the secondary weapon can have seriously differing behaviors (including a giant buzzsaw-type arm).
Oh, and don't forget the bombs. Above your level indicators are little bullet looking icons that indicate how many bombs you have remaining. Since there is no designated "bomb" button, the way you launch one is to press both the secondary weapon rotating buttons at the same time (the 1st and 3rd buttons). Do that and....Kaboom.
One way to score extra points is by collecting the little black/gold shields floating around that have little roman numerals on them. The first one is worth 100 pts, then one with "II" floats around and it's worth double (200). Each time you get them within a certain amount of time, the next bigger one comes out and is worth double the previous. I've seen them worth up to 6400 pts if you keep collecting them quickly. Otherwise, they disappear for a while and restart at 100.
There are sub-bosses and big bosses. And they come with timer/countdowns to make sure you are aware of how long it is until your ultimate doom.
The game's graphics are modern and busy. Some of the enemies look awesome. They are big and complicated. Sometimes the terrain is very busy and I have run into things that I did not realize that I had to blast out of the way before I encountered them. Remember, you are on the ground in a tank, not flying in space.
With a cool rotating secondary weapon, sweet graphics and full-screen action whether scrolling vert, horz or diag, Guardian Force is a good time.