Either from the sky or from under the ground. Once Hong Dali said this, You Muming and Kong were immediately impressed.
He could even think of this, this was practically invincible!
They pondered carefully for a while and agreed that this was the only possible way too. If the enemy wanted to forcefully attack them, they definitely needed siege machines to do it. Other than that, the only possible solution was to dig a tunnel under the fortress. After all, on this battlefield, even those ability users with flying abilities wouldn't be able to fly too—if flying was possible, what use was there for strategies and tactics? Just fly up into the air and attack the enemies would do, right?
Since flying wasn't possible, they could only dig into the ground. And obviously, insect-type ability users were experts in digging.
