#1 Is to deal with spiders and occassionally the enemies from point 2 that bug over the wall.
#2 Is seems to be important to put the wall at the bottom of the stairs. otherwise, the enemies just go around it.
#3 With the defenses where they are they kill the wyverns without any extra help. Positioning is also important so the Summoners don't turn and target them.
#4 In the later rounds I found moving these out a little was needed to kill summoners quicker. Otherwise they would pelt the core a little before dying.