You need to add upgrades for the towers, preferably ones which are cost efficient (for example, double the damage per second for less then the cost to build another of the same tower). Also making the poison/ice etc towers have their effects last forever would be better. Its frustrating seeing the poison wear off when the creep is about to die.
Apparently 130 coins is not enough for the 'Earn 100 coins' badge. Maybe counting the coins on loading a save instead of just the ones earnt in current playthrough?
21 is easy, look how long each piece takes to go back to the outside. Bottom right is the slowest, followed by bottom left, then top right, then top left. Click them in that order fast.
How about a checkbox on the rate map screen to report a broken map (eg no starting survivors, or no land at all), with a high enough ratio of checked vs unchecked ratings the map is automatically deleted. Such as 2:1 (so minimum of 2 people have tried it).
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