I'm not sure if this is a glitch/bug or not but if you refresh the page or open the game you'll receive 1 minute of passive gold bonus even if you weren't logged off for a minute. So if you refresh the page and you're not on the game for 5 seconds, you'll get 60 seconds of afk gold.
 "Defender Idle: New Beginning"){kind=icon}
 "Get A Little Gold"){kind=icon}
It can only be unlocked in Tower.