I have a fast Question: What do the DEF points really do to a unit ??

Lets say I got 100 in Attack
Defender got 200HP & 30 in Defens

Im only sending 1 vs 1 troop
shall I take 100-30(def) to se my damage?

Then It will take 3battle rounds before the enemy is dead, (3x70 in damage) is this right ?