SAT Invaders is intended as a mid-level demo, with menu handling (through TransSkel again) and levels, but no scores or special options. This makes it much smaller and easier to understand than HeartQuest.
SAT Invaders uses the simpler collision detection scheme, since all sprites are either friends or foes.
Special features:
• Menus, event processing
• Sound and direct-to-screen blitting can be turned off
• Menu bar hiding.
SAT Invaders might be a possible platform to build on. Note that it doesn't save its settings. For optimal compatibility, you should save settings and scores in the system folder. (See Preferences.p in HeartQuest.)
IF YOU LIKE SAT INVADERS, you should try "Space Invaders" by Bettini Simone! That game is developed from ST Invaders, turning into a full-featured Space Invaders clone - the best I've seen for the Mac so far!