Objects are terrific for games, especially games with lots of little enemies, because you can easily define different looks and behaviors to each one. It's especially fun once you've got the base logic in place, because you can then try out some of your ideas and see how they work, and you get instant feedback. This was the case for Zap Pod! After getting the base stuff going, I added a half-dozen different alien behaviors in a couple of days. It made the game richer, and it didn't require much extra work. Seeing your ideas come to life is the most rewarding part of the process.
Following my earlier model, I'll probably have an "enemy" base class for all meanies. Derived from that will be "marcher", which is an unintelligent critter that just walks back and forth, like a good invader should. Other aliens will simply inherit behaviors from "marcher". Also derived from enemy will be the saucers. I'll probably have a couple different saucers just for fun.
For graphics, I'll probably leverage a lot of the critters from SpriteLib. The author's got a lot of cute little aliens and space ships that fit the bill perfectly.
As for alien properties, I was thinking of the following for starters:
- Standard marching space invaders (level 1)
- Aliens that drop out of the sky when you shoot 'em, forcing you to dodge them after killing 'em.
- Aliens that "get sick" when you shoot 'em. You've gotta shoot 'em again before they'll die. If you don't, they eventually recover.
- Aliens that try to escape if you shoot 'em. If you shoot 'em before they make it off the screen, you get bonus points.
- Flying saucers that replenish the aliens if not taken care of quickly.
As always, some of these won't work out, and some new ones will appear as the game evolves. I'll keep you posted.