I just reached lvl 40 and my castle is lvl 5/6 split, so I still consider myself new to the game,
I clicked on a place on my inside castle wall and discovered some hidden gems.
the wall sort of squeaked when i clicked on it, a few more clicks and 3 gems appeared
just like coins above a tavern or bread above a barn.
I took a screen shot and pointed an arrow to the spot that squeaked.
from now on I am clicking on things inside my kingdom more often