Sieging question

[GrudgeBringer]

If I am being beieged in a fort and I drop in 2 or 3 castors of Rain of Stones.

1. Will it hurt the forts garrison if I DON'T have it set to break siege.

2. Is it considered a Magical attack and will hurt the Eterel beings.

If I don't kill them all and set to retreat, they SHOULD go back to a friendly province (if adjacent) and NOT the castle garrson.

As long as I am asking unusual questions...Will the robe of Calius protect agianst ..king of Banefire or that type of attack?

It says protects agianst all elements and Magic so I wpould assume so. Never ran in to this mutt before....grrrr

Thanks