Technically archers are useless except in the very first lvls beofre you have mages. So do not bother upgrading them. After use a "sandwich" strategy (one layer of canons, one layer of mages, one layer of foots) it gets you through most lvls. Last one can be very frustrating as some waves are very close calls, if your towers inflict a bit less damage you let 3-4 enemies going through.... randomness of the things can be infuriating...
Really good addictive game. One comment though the navigation into the sub elements to craft is rather painful. Having somewhere the path followed to allow you to revert back to a higher item would ease the navigation.
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