I have to say that, while I managed (by accident) to basically "cheat" at this game, until I did the game was very tough, and quite fun, and I'll get into my criticisms after I explain to people having trouble the way to win, step by step, although I would like to mention that this works best after one island is unlocked, but works fine without it.
Step 1) Run to the nearest safe zone, collecting resources along the way
Step 2) Go to another island and make sure at least one zombie spawns there before going back to the middle island.
Step 3) Hide near the safe zone and shoot and hack down as many zombies as you can before they start grouping up
Step 4) Ground slam WHILE STILL IN THE SAFE ZONE, stunning all the zombie near the gate, walk you, attack ONCE, run back in. (If you attack more then once you risk
Step 5) Repeat step 4 until they are all dead.
Step 6) Collect all the resources on the islands, ground slamming the single zombie left alive to pass by him and collect resources on his island. When boxes stop spawning kill him to finish the level.
If you have your weapon system online, do steps 1 and 2 then just hide in the safe zone until they're all dead. I don't know how long this takes (it happened to me when I went and played a different game with some friends and came back to all of them being dead) but it will probably take a while.
It's slow, but it works really well.
Anyways, for criticisms, I was kind of annoyed I couldn't CONTROL what was being upgraded, and that the weapons on the castle didn't do much damage. Also I can't really imagine how people are supposed to win besides using the method I just described, which, to be honest, isn't very fun, only effective.
Still puzzling OUT that strategy was fun, and having it work was fun, and the ending was cute too, so bonus points for that. I feel like the stomp should have been explained more, since I had to look in the description bar to know how to do it, and I didn't notice the stomp bar until about... 7 levels in? And explanation of that would have been nice :)