Oh, OK. I don't know why I was thinking 12', but I was admiring that as the fencing around my own backyard (the parts that permit fencing) are only 6' high, and hey have to slope down to 4 ft. with open pickets, so as not to obstruct the neighbor's view of the "wilderness area" behind the properties.
As for the "barking," I was concerned because you had mentioned that if people see a Dobie on your property, they'd try to kill it by throwing poisoned food. So, if they can't see it from the fencing, might they still be able to hear the dogs bark and attempt to kill them anyway?