I believe the ability for zombies to climb into the cabin is cool but not implemented properly. Previously in the game metal sheets would stop zombies from stacking up and climbing into the cabin letting you save ammo and heals waiting them out, but now if you attempt to wait out the zombies in the cabin even with metal sheets covering the windows they will phase in this is not how it should work. if the devs decide the zombies need this ability then they should make metal sheets prevent zombies from climbing in but they have an hp bar so there's more to spend money on as metal sheets need to be replaced and you can save the ammo waiting out the zombies as long as you keep your metal sheets in good health.