Good, good.
But, like Tashpiro said, you can stay on level one forever, just don't build any trees, build and upgrade your towers and weapons, and your set. All it takes is time to beat the highscores. Also, I found it got incredibly hard on the last level, and it was incredibly easy between levels 5-9. I was getting through with just two catapults (upgraded to level 10 dmg), two towers (upgraded to level 30 land and air) and 3 upgraded totem poles (plus large trees around the edge of the island to act as shields). After level 9 though, I got hit by a sudden rush of about 20 boats, most of them overlapping, so it lagged alot, and I couldn't kill them all.
Anyway, it's a well made game, besides the level 1 tactic, which I suggest could be fixed by simply starting with a tree, and if that tree is destroyed between the 1st and 3rd level, you lose (since the trees do not regenerate until you get totem poles, the tree would eventually die if you didn't attempt to build and level up). But, it kept me entertained for a good half hour, so I gave this 4/5, 9/10.