New castles aren't given Green Thumb now, but the game has a task to upgrade skill 2 of Green Thumb...

You will need to recruit green thumb, this will be a random pull from the normal hero recruitment at the Bar. It may take a long time but eventually you can get him.

Once the hero is recruited you just need to unlock the second skill which will mean leveling him up to 6 and using some wisdom medals. Open the hero menu and tap on the green thumb. Then select the scroll icon at the bottom to see the skills list and tap the second skill.