“Of  course  I  considered  the  possibility  of  the  enemy  creating  a  tunnel underground  to  invade  us.  Therefore,  I  made  sure  that  the  Great  Iron  Walls extended underground. The walls are fifty meters aboveground and continue fifty meters below, making them one hundred meters tall altogether.”

“D-does that mean they went even farther underground and passed under the walls?!”

I hear Seiya clench his teeth.

“Just in case, I made sure to make the last wall extend one hundred meters under ground, twice as long as the others, but...”

***
 
Volume 4, Chapter 32