The problem with the game is that there's no control. I can't tell my units to stop or where to go, so they end up running head first into a group of enemies they're weak to, rather than waiting for back-up.
Ah, I found the problem. If your profile is set to private the game won't collect friends from your list, so you won't be able to progress without cash.
I'm a little disappointed that friends are required to progress through the game. (The brambles, more specifically.) The friend system doesn't seem to function correctly, or I'm doing something wrong which wouldn't be surprising. Great game otherwise, I just wish friends weren't required.
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