Very enjoyable game with great performance and also a pleasing pixel art style :)
I like how you added the different types of enemies, this definitely made the game more fun. But I would really like to point out some of the small downsides from it first:
- At the beginning of the game I thought that the bar with the head was like the health bar/armor and the one below the damage/amount of shots we can do, but later I understood the concept of the game a little bit more. You just have to protect both of the buildings on the sides, but the only thing they're different from each other is just the look, the don't give like an advantage or something. I think it would be a little bit more fun if you made so the one gives you more damage depending on how full the bar is and the other one like the speed or something? ;)
-Speaking of advantages, I think the game would also need some upgrades or bonuses in the fight, like higher fire rate, burning character that kills enemies on contact, a clone etc.
Maybe also a 2 player mode then with 4 different buildings? Would be awesome ^^
-I also think that pretty much all of the Ludum Dare 39 are amazing and I love jams like that, they deliver to us very high quality products that are a blast to play and also an interesting competition. But from my own experience I would say, that the games are a bit too "straight forward", I mean, yeah, they're all a bit arcade-styled so that they can focus on the main goal, but I think that the game creators should also add some more customizations to the game, like different stages, the option to customize the character etc. It's not too bad if you don't do it (most don't do it), but I just generally think that those would be some nice touches people could work on after they've released the game, just to make the games more in-depth :P
welp, yeah, that was pretty much it,, hope you'll think about changing some of the things in the game that I've mentioned ;)