Wow. Just... WOW. This is by far one of the best LD games I have _ever_ played, and I've been doing this since LD36. It plays really well, it has great graphics, it's fun and engaging, the premise is interesting, it has meaningful progression... There's really nothing not to like about this.
So take these suggestions as a way to really push this game to the next level, not as criticism against what you've built, because this is something that could really be awesome:
Some good sound effects would really make this, although given how many entities there are you'll want to be careful about it (and definitely have the sound drop off pretty quickly with distance).
+1 on being able to cancel cannon construction, and I think I agree with the suggestion that it would benefit from having a puzzle mechanic of having a limited number of cannons. (And if you have that, then of course you'll need the ability to move or salvage a cannon once you've built it.)
The lava is a bit punishing. I would suggest moving the player to safety when they hit the lava because right now I tend to just get stuck bouncing around in the death pool until I run out of lives.
Having some prebuilt stages for tutorials is good, but you might consider doing some procedurally-generated worlds in order to increase the game's replay value. I'm open to providing some assistance in that regard if you want some help; procedural generation is something of my specialty. (Of my nine Ludum Dare submissions to date, seven of them have been procedurally generated... well, technically eight, but The Dungeons of Indianapolis doesn't count because that's more like procedurally copy-pasted. :stuck_out_tongue:)