How do I make a defensive line?

I did the tutorial but that confuses me, I don't know how it works.

Prispeval MegaPrem, 19.03.2020 at 18:02

I did the tutorial but that confuses me, I don't know how it works.

I would like to make a question... defensive lines always break if attacked? even if both sides of the line survive?

Yes

Would not be better (and more realistic) just break if one side is completely killed?