What determines who you take as a prisoner?
Solution 1:
Although many say it's purely random chance, from my experience, it seems you need to perfectly kill them. If you can get a defender's health to exactly 0, it will allow you to capture that defender.
For example, if a defender has 20 health left and you do 30 damage, they die. Otherwise, if they have 20 health left and you do 20 damage, they're just knocked out.
This isn't confirmed, however.