It's possible that the player ran out of blocks. I guess somewhere it tells me how to get more blocks. I thought destroying mountains might do it, but it didn't.
The buff may well have been explained in the rules, but you made the classic mistake of assuming your player will read them! I tried mousing over the indicator to see if that would tell me what it did, but it didn't.
By "turning turtle", I mean that if all the players play defensively, nothing happens except that when you attack you lose.