At first I was just wondering what the point of this Monster Gate is. Then I thought it might be a bug. Maybe you're supposed to fight before you can get the key behind it.
(There was a key to the right of it, I only saw the Gate after I ran around the top path. So I dumped the key ahead and came back to make the screenshot. Invisible so I don't get eaten while doing /clear and getting a good spot.)
I went along the top path first (from west to east), it wasn't very narrow, I ran past Zombies along paths like that all the time that day.
I saw the Monster Gate wondering what that was about, grabbed the key, then went back the path I had come from (now going east to west).
I dropped the key, had another look at the gate still wondering about the point, retreated a little, went invisible, did /clear, then went back again to the gate to make a screenshot.
I don't see where there's any kind of trap.
You kinda answered yourself there.
First you go thru that path as a sort of mini-shortcut, but when you grab the key, you are forced to take the much narrower hallway at the top, in wich you're more likely to get hit by zombies.
It's... a trap. Indeed kinda small and meaningless but does it's job.