Attacking armies must defeat your army to plunder any resources not protected by your storehouse.
The army must get past your wall and its defenses to plunder your gold (storehouse doesn’t protect gold).

Your Marshal’s combat score is used for armies that are on defense. 

If you hide your army “in sanctuary” (done in castle) then it won’t fight, but the wall defenses will fight either way.

You city loses 1 happiness if your army is defeated (or absent) and 5 happiness if the walls are defeated.