1: Because the cannonballs hit them of course.
2: I don't know that one.
3: If you dropped an explosive crate, you should see a little box popping up when you stand close to it, it should say ''ignite!''.
If you want to build something, select the hammer, hold down f and start hitting the object. When you want to dig a simple earthwork, select the spade, hold down f and start hitting the pile of dirt.
Hope it helped!