Originally Posted By: frog51
(me too... that large yeti that freezes your towers was a problem that required some time...)

A little tip in general that they don't seem to really indicate in the game: you get stars for beating the Hero and Iron challenge modes on each level. You get one additional star for each completed challenge, and upgrading your stuff is clearly essential. They certainly don't let you get too far ahead in the game for each progressive level, that's for sure...
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Matt