Nice work!
I like how you used SDL and OpenGL, how the game is tiny (just 200kb without the libs!), and how editing maps is easy.
Minor things:
-the game window is a bit too tall for my small laptop screen, although it's just playable.
-the sounds are a bit harsh. Some low pass filtering or reverb might help. How did you make those? (bfxr/sfxr?)
My main criticism:
The physics is buggy, especially for high speed shots. There's tunneling (the ball moves through walls) and collisions being resolved in the wrong direction (wrong velocity reflection).
How did you implement it?
I think the best way to do it is to implement continuous collision detection and resolve between a circle and a rectangle, where the rectangle consists of four line segments and four circles with radius zero (the corner points). Continuous circle/line segment collision detection and resolve is easy enough, especially if all line segments are horizontal and vertical, like in your case. Perfect continuous circle/circle collision is a bit harder: that actually requires writing down a quadratic equation (for the distance between the circle centers, parameterized by time) and solving it with the abc formula. But that is what's necessary for perfectly resolving high speed collisions for a ball that bounces on a square corner...