-1 star for no quality settings, or fast forward button.
Its extremely laggy on levels 50+ so a quality option would have been great. I would even be willing to throw out all graphics and just use cubes instead of monsters to get rid of lag. Im willing to get rid of all bullet animations, explosions, mob skins and all for smoother game play, that would be a great option.
The first five levels are easy compared to the last one, and i would have liked a x2-x4 speed button (i would imagine it would lag pretty bad in its current state at X4 speed so reduce graphics option is crucial for this to work)
The first 5 levels are fun and some took me more that one try. I would love to see more levels, i could play this for days.
The last level is extremely challenging and i think it is great to have as a last level to really give the player replay value. If i was able to beat the last level in my second try and have the whole game be over i would feel that there is something missing. The last level is more of a survival mode which i loved.
as @mariaReine said the air troops become insane after level 70. In order to keep up with them you need MORE than one maxed out anti air weapon per flight troop wave. i died around level 75 with 9 maxed out anti air troops in the center and it wasn't even close, those planes just flew right over them and killed me with more than half their hp to spare.
for those getting stumped by the last level, I haven't beaten it, but my strategy to get to level 80 is to keep in mind you HAVE to make them turn around in order to be successful. Make two long paths, when they reach the end of the first path, block it. They will turn around and head back to the other path. When they are at the end of that path, unblock the first one and block the second path. You will have to repeat this very often and i found that having those AOE sonic turrets where the 2 paths meet is the best way to deal damage because there will be over 50 enemies on the map at the same time. To get the ball rolling, group both spawns with walls into a corner, make as much of the first path as you can with the 5 gold towers, and make a little bit of the second path in case they reach the end. The tricky part is saving up 50 gold for that first anti air unit before they attack. once the first air wave is complete and you have over 15 lives things get a lot easier. Just remeber that you will need A TON of anti air units maxed out. I mean A TON. If you want to win i would aim for around 2 maxed out anti air units per air wave, even then you still will probably get over run. Also try thinking diagonally, my strategy involved most my paths running diagonally.