Added simple collision detection, varying enemy strength (1 - 5 "hit points") and enemies dying when they reach 0 health. I have the beginnings of the player being hit, but I did something funny earlier in the development cycle and now the visible ship is actually about 20 pixels farther out from where the code expects it to be. Probably something with the way I embedded it in the first place.
I think I am going to put a "Formation" class between the game class and the enemy class. The formation will take care of all of the enemy behaviors and keep them all playing nice. This will simplify the code for the main game class and allow me to do things like create bosses which are actually aggregates of several enemies of different size and shape. I think this is essentially how other arcade games (cf. Raiden X, 1941, etc) handle bosses which gradually lose pieces as they are worn down.
So next up: formations, player death, explosion animations, and perhaps the beginnings of a game UI.