Totally agree on the gameplay balance stuff -- definitely ran out of time to make a proper difficulty curve. The amount of cannons that spawn in does actually change from level to level, but it should probably be more of a gradual thing.
The levels are somewhat randomly generated (Very simple procedural stuff, just picking from a list of positions) which was intended to lessen the frustration of dying, but I feel like there wasn't enough variation in the gameplay to make it worthwhile to restart.