That is for wall traps, a feature that has not been released in this game.
Zara
said
almost 7 years ago
Thank you for getting in touch with us!
At
the moment the Trap Building feature is not available but will be quite
soon even though I can not disclose an exact date for when this will
happen.
However,
you can assign Skill Points to Tank Attack 2 even if you don't have the
Skill points on Trap Building. If you have enough Skill Points, you can
try it. You should be able to assign them to Tank Attack 2 as we have
already calculated so that the Trap building won't affect the other
Bonuses.
If there is anything else we can assist you with, please let me know.
yasser darwish