One of the earliest computer game genres is that of the "artillery game," in which players (generally two in number) take turns adjusting the angle and power of their respective projectile launching device, then letting fly while the other guy cringes. This type of game goes back so far (no one is quite sure when it started) that the early ones didn't use graphics or, in some cases, even a monitor screen - the program communicated the game's progress via dot matrix "hard copy" printout! Not very "green", but a good way to ensure your game was saved for posterity (though who would save such a thing?) You can get an excellent idea of the game play of one of these Ur-tillery games via this BASIC listing in the Atari archives.
Wendell Hicken breathed new life into the genre with Scorched Earth for the PC in 1991, and Michael Welch spread the fun to the Amiga a few years later with Scorched Tanks. Mike released Pocket Tanks for the PC in 2001; as fate would have it, shortly after the terrorist attacks that year. But so-called "Tank Games" have never been about violence - at their essence, they are mathematics and physics in a fun, lightly tactical turn-based challenge. Some versions have dialed down the potential carnage by introducing scenarios such as two gorillas throwing bananas at each other, or Worms throwing sheep at each other, etc. (Turning on "wind" makes it much more challenging, you'll need to adjust either your power or angle before every shot, if the wind speed or direction changes.)
Artillery games are still being actively developed. Some have expansive 3-D environments. But stick with the simple 2-D versions. Sometimes the old ways are the best ways.
Wikipedia has some pretty good info on this topic, and "Scorched Parabolas" by Matt Barton is probably the definitive piece.
If you want to play a single-player artillery game right now in your browser, you can do so thanks to the Discovery Channel.
