My thoughts: would be good to have some kind of a notification like "chest has been taken" when on the second island just to know that they took it. I ecountered a bug where the pirates instead of taking the chest to their ship instead push the chest back to its place and then get stuck at the end of the path. 
(In my opinion) the attacks should be separated into waves and the map should inform you whether the wave has started or not. there should also be a total number of waves displayed somewhere.
Like the music a lot, the 8 bit/pixel style is deffinitely nostalgic which makes the game look well in a way, kinda like mario.
An interesting idea could be to allow the character you play as shoot as well which would make the decision making between eco and defense even more complex .
Overall,preatty good for a early version of the game, remember though that tower defense games are very popular making it a quite a competition to create something unique but i think this has potential.