Fun Game!
I disagree with the reviews below- this game is, in fact, not as difficult as they say, but I think I understand why they are having trouble.
To the author and to players: Here's the deal- the simple cannon, which is suppose to be the "bread-and-butter" weapon, is simply completely ineffective. It simply takes way too long to charge up its power. In my opinion, the developer of this game should redesign the cannon completely in order to be actually usable, because as it is now, the weapon just doesn't work. Don't use a "power+angle" system, because the targets just simply come too fast for that method to work.
To players: As the game stands now, in order to do well, you need to buy a DOUBLE machine gun RIGHT AWAY. It should be the FIRST thing you buy.
The only thing I used the cannon for was as a tank to protect my more delicate double machine gun and secondary weapons.
I've gotten through the ocean and mountain campaigns, and I'm about to start the third one. Here's the build I've used:
2 cannon "tanks" to absorb hits in vulnerable places, a missile launcher, a double machine gun.
The cannons should ONLY get health upgrades and not be used in any other way but to absorb damage. They can take a ton of punishment.
The missile launcher should primarily get reloading upgrades until its reload rate is sufficiently fast, and then you can consider damage upgrades, though they are probably not that neccesary. If things are placed properly, it should not receive much damage.
The machine gun should primarily receive reload upgrades. While the damage upgrade would be incredible, it's simply too expensive.
Whenever the reload time is up, switch over to the missile launcher, launch a missile, then switch back to the machine gun. If you're anticipating a big enemy like the nautilus-looking ones, you might have to time your missile launches, especially if you don't have many reloading upgrades yet. But once you have a sufficient amount of reload upgrades, you can just keep launching missiles whenever the reload time is up.
Again, very good game overall, but what really holds it back is the poorly designed cannon.