Although it's very primitive-looking, all the elements of the original Star Castle are present. The rings at the center of the screen can be pecked away at with your ship's gun, but when there's an opening the turret at the center (your ultimate target) will fire a powerful shot at you. The turret also dispatches buzz bombs, which stick to the rings and ride them around a bit before emerging to chase your ship, and you can try to shoot them down as well. To make this a complete game would involve wrapping the actual play in a level-based structure -- keeping score, resetting the playfield when the turret is destroyed, and increasing the difficulty with each step by speeding up the rings, turret, and buzz bombs.
It's not finished, but nonetheless I'm kind of pleased as the engine code driving the game is pretty robust. There's a genuine physics system which takes the hassle of managing object collision and motion off gameplay's hands, a nice input system for handling all keyboard input, and solid if simple graphics and sound systems. The result of this is that actual gameplay development was pretty easy -- to take an example, the buzz bombs didn't go in until yesterday, and it only required a few hours to get them working.
So I'm going to consider this a moral victory. I may try to develop this a bit more -- my original design, before Fallout 3 interfered with my schedule, included a wider variety of enemy ships, powerups, and all the usual nonsense. Alternatively I may consider what other games could be developed based on this code. Or alternatively I'll be lazy and do none of that. It's always an exciting cavalcade of adventure around here!