Unlike other supernatural undead creatures in fantasy fiction, there is no clear consensus as to the properties of zombies. Unless you are reading Stephanie Meyer, vampires are always pale night dwellers who drink human blood and can be defeated by a quick stab to the heart with a wooden stake. They generally represent things like sexual desire, coming in through the windows at night and draining people of their innocence. Zombie, though, are much simpler and thus more problematic as metaphors. As the title of the above mentioned comic and TV show simply puts it: zombies are the walking dead. Reanimated corpses that simply consume until their brains are completely destroyed.
Beyond being dead and needing a brain, there is not much consensus as to what a zombie can do. Some can run, some just stumble along; some have increased senses, some are essentially mindless killing machines; some are unnaturally strong, some can be wrestled down by an average man or woman. And this is not necessarily across media, but within one work.
For example, take arguably the most popular piece of zombie fiction, AMC's adaptation of Kirkman's The Walking Dead comic. In the first season, some of the original camp has made their way to the top of a building in Atlanta (a department store, if I am not mistaken). There are two sets of glass double doors between the Rick and company, and the horde of undead (and it is a horde) outside. Eventually, the horde pushes through the doors and gains access to the building. It's not shown exactly how they gain entrance, but it can be assumed they pushed hard enough to either crack the glass or to rip the door off it's hinges (or break the locks, though that seems less likely, since locks are meant to precisely not do that).
Later in that same season, when T-Dogg (may God watch over his soul) dropped the keys to Merle's handcuffs, leaving him trapped on the roof, he barres the heavy metal door with a chain. When the company returns to the roof to rescue Merle, the chain remains unbroken and the door on it's hinges. Apparently, these doors are more difficult to break. Or Merle, who cut his hand off, escaped before they could. When T-Dog was explaining himself to Daryl, there was a note of concern as to how well the door would hold up.
In the second season, Lori flips her car when she madly searches for Rick, Glenn and Hershel. While passed out, a couple of zombies find her trapped in the car and attempt to make a meal of her. One finds a hole in the tempered glass of the windshield and pushes so forcefully against it that his face rips off and the glass starts to shatter further. Again, this is not just any glass, but the treated glass used for windshields, which is meant to take quite a beating. That the zombie has pushed through is fairly impressive, not just as a feat of strength, but also a sign of how determined they are to reach living flesh.
In this season, what remains of the small groups has made their way to a prison. Behind two chain link fences, the survivors can leave in relative safety from the walking horde of ever-consuming zombies that stumble around the edges. This, though, is problematic: the zombies have shown that, as a horde, there are few things that can stop them. Their hunger for flesh supersedes their pain tolerance (which as dead things, is remarkably high). They will just push and push and push until they get where they want to go. Especially if there is something tasty living thing to eat on the other end. It would seem to me that the horde would bunch up at the entry point and the sheer mass of the zombies pushing against the fence (a chain link fence, mind you, which does not secure the doors deep into the ground) would cause it to break. The glass of the department store and Lori's car window couldn't stand up to the walkers, but suddenly a chain link fence can? As this video shows here, even the weight of one heavy set teenager who makes bad decisions in his life can bend the support posts of a fence. The combined weight of several dozen zombies unceasingly pushing against a similar fence, even one fortified against outbreaks, would eventually bend.
Also, in the prison, Rick and the other survivors manage to lock the fence using carabiners and chains (not particularly strong chains at that). At one squeeze point in the prison, there is a sliding chain link fence (probably the weakest of the fencing options) held closed by just such a contraption. Now, assuming they did find climbing grade carabiners in the zombie infested wasteland of Southern Georgia, these would be able to hold, for brief periods, around 21 kN or 4,725 lbs. As this article from Boy's Life notes, though, carabiners are not made to withstand constant pressure and are more for safety sake, such as when rock climbing. In the prison, these are as good as fortified steel doors, tightly padlocked shut.
Which, then, is it? Are zombies so focused on eating humans that they will literally walk through doors to get at anything alive? Or can you simply hold them off with a good fence, some chain and some carabiners? As metaphors, they certainly can still be useful. What The Walking Dead needs to remember is what their zombies can do. This is why the Twilight books upset so many vampire fans: they bucked convention when it complicated the plot. That is, it would be hard for Bella to meet Edward in high school if he couldn't go outside in the day time. In a similar vein, if a horde of zombies can push through anything, then they shouldn't be stopped simply by latching a gate shut.