I'm not sure if you're aware of it, so I thought I better share it: Whenever the dragon flies on "Beresad's Lair", you can click his gold. This gives you 100 gold each time. Pretty helpful.
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A few of the hidden achievements if you’re having trouble: The singing frog is at the lake of “Sape Oasis.” The mermaid appears in the upper left of “Buccaneer’s Den.” The SOS bottle appears in the river of “Snapvine Bridge.” The alien hunters can be found at the upper left of “Crimson Valley,” the upper right of “Snapvine Bridge,” and the upper middle of “Lost Jungle.” Dr. Walton is the monkey that appears in wave 6 of “Temple of Saqra.” Click the runes below: middle, left, right. Hope this helps.
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 "Kingdom Rush"){kind=icon}