Seems like the offline gold is bugged for me, it only works sometimes when I am on another tab. If I turn off my comp and come back the next day, there is no gold earned at all.
If you leave the game running it will still occasionally get resources from the browser to check in. The only way to reliably go offline is to close the tab.
Most guides are a little outdated due to the game having had some updates along the way. As usual, stick to upgrading lvl 6 towers (rotate through all the colors). For level 9 and 10, don't worry so much about score as it'll most likely be gold medal as long as you complete the level. Also for level 9 and 10, from round 60 onwards, build about 10-12 teleport (keep it at level 3 and put clovers in them). For those immune to teleport, build a new lvl 6 tower to counter that element. Otherwise, most of them will get teleported back and it'll be easy getting that hard badge.
Tip : To get all the badge with ease, complete all 15 level first no matter how many stars or whether u complete the special challenge
Then, the easiest way to complete special challenge is to put all 1 star
( u would have completed the stage a 2nd time )
Next, repeat the same stage but put all to 3 stars ( Completed the stage 3 times)
Note : It is hard to do the special challenge and 3 stars for the later levels
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If you leave the game running it will still occasionally get resources from the browser to check in. The only way to reliably go offline is to close the tab.