Or you know, since Merlin made it as a mortal wizard using mortal magic, he shaped it like a pentagram cause that's the most powerful symbol of magic under mortal command? Island is a big summoning circle/ holding pen after all.
My internet is cutting up a little, I really didn't see the pentagram reference at the bottom until too late. Perhaps it should be given its own power point? I mentioned this when I was saying Harry would use such a spell in time space to call his own timeline, but everyone was confused by the six timelines with five points lol.