While everything is there on a technical level, the challenge and rewards are just not balanced properly. Maybe it was because I chose to play on the first map, but my recipe for success was to build a small shack, then farm for a medium shack on the other side of the map. Once I had that, I farmed for a largest shack on the lower island. Once that was done, I farmed for a minigun(because the bazooka didn't appeal to me)
Once I had those 3 shacks and the minigun, I ran around and killed as I pleased until i got bored and jumped off the level till I died. I built a hospital for the heck of it, but never used it. I didn't even need the other buildings.
I think that you should give the soldiers the ability to move from island to island and through the pipes. I understand that the idea is that they're supposed to be stationed on each island, but honestly, there's no real reason to. You should also give the player the ability to demolish or change the buildings to a new type so that you don't have to rely on mobs destroying them. Finally, I'll suggest having a limit to how many soldiers you can have of each type, not a total limit. This way, you can have an equal number of all types of soldiers and you won't have to worry about hitting that 40 soldier limit on one of the islands, leaving the others barren.