Chap

Hello. I'm building a military fort our servers horde base and I'm using the 7x4 Roll Up Door as the main entrance for it. The use of the base is to be a horde base where you open the door and let the zombies flood into a maze like structure and you fire down on them from the elevated wall positions and catwalks. Well as I've been building I've been noticing no zombies coming into bother me at all. Even with the main door wide open. Well I did some testing tonight and I found out that they can't GET in. They all just go to the center of the opening and start beating on air like something is blocking them. The only time they can get in is if one stands on the head of another and they jump over. It seems like there is an invisible barrier that's about two blocks high. Not even Spider Zombies can get past it. Plus they are wrecking the door even though it's wide open. I assume this isn't intentional?