this is a good game, but it need some balance work, the scattered way that flying enemies enter the field means that at least the early waves need lower hitpoints. As it's virtually impossible to have sufficient defences to take them out, without screwing yourself over for when the first boss comes
Shark boss single handedly makes this game suck. On hard difficulty it's literally impossible to stop him from destroying your entire base in one run, while using all your power ups and firing with your maxed out flack cannon.