At a forum member's suggestions, I took a look at the (now abandoned) Basic implementation for the Atari 5200. It's pretty quirky, but it can still be used for simple games.
I wrote a very simple shooting gallery game which demonstrates concepts such as objects, missiles, colour, input, collision. sound. Guide your "gun" with left and right, and shoot by moving the joystick upwards. The three targets respawn off-screen once shot, so you can keep shooting them for as long as you like!
When developing your own game for the Atari 5200, be mindful that having tab characters in your source code may cause the compilation to fail.
the Shooting Gallery rom. Should play in most Atari 5200 emulators
the Shooting Gallery source code. Use it with the 5200bas dev kit
the 5200bas dev kit. Unzip it to C:\. It comes with an emulator, and the source code for Shooting Gallery, found right inside the 5200bas directory. Shooting Gallery source files are prefixed with sg_
a good Atari 5200 emulator. I used it as a run-time environment to try out my homebrew code