Its a little confusing as there is 2 17's used
Ranged are -17 Strength
Melee are 17% of damage is applied
hopefully the table helps
View attachment 474328
We can see from this diagram that a crossbow still gets the -17 STR with a tower but applies 100% damage against the garrison.rather than reduced by the % of wall left
Naval Ranged do not get -17 STR
View attachment 474329