Guarding The Portal
There's some specific region of dimensional weirdness—possibly a Hellgate, though it doesn't have to be malign. In any case, strange things regularly come out of it, or are attracted to it, and at least half of them are dangerous somehow. It's the job of Our Heroes to hang around near the weirdness, be all "None Shall Pass" if needed and deal with everything related to it. As a consequence, in serial works, it's the standard justification for the show's Monsters of the Week. The weirdness may have caused the surrounding area to become a City of Adventure.
- This is used to explain where E-123 Omega was during most of the Sonic the Hedgehog/Mega Man: Worlds Collide crossover. Rouge asked him to keep an eye on the portal back to Mobius in case Eggman or Wily tried anything funny with it while she went to save Shadow.
- At some point in the Wheel of Time series, Rand sends guards to each of the known Waygate locations in order to prevent their use by the bad guys to travel quickly around the world.
- The Outer Gates from The Dresden Files are where the Outsiders - beings from outside the universe - can get in. They invariably have destructive intentions, so it's necessary to guard the gate at all times. With an army.
- In City of Bones, what Madame Dorothea is really doing is guarding a Five-Dimensional-Door in her apartment.
- Buffy the Vampire Slayer is about guarding the Hellmouth.
- For series 1 and 2, Torchwood was about guarding the Cardiff Rift.
- Star Trek: Deep Space Nine is about guarding the Bajoran Wormhole.
- Stargate SG-1 as a whole tends to take place on the other side of its portals; it does have the occasional episode which uses this kind of plot, though.
- And the various locals and Jaffa garrisons they run into could be seen as doing the same thing, with SG-1 as the weirdness.
- Any time more than one team is sent through the gate one it told to stay and guard it.
- Stargate Atlantis is a borderline example, as they're actually guarding the portal from the inside - they only have one door back to this galaxy, and everybody's trying to take back the territory it lies on so they can get through it.
- Primeval is about guarding temporal rifts in the South of England.
- It's Up to You to guard and shut down the Oblivion gates.
- In World of Warcraft, the forces manning Nethergarde Keep watch over the Dark Portal.
- World of Warcraft fit this trope back in vanilla WoW (before the release of the first expansion, The Burning Crusade). Now that Outland is well-trod ground, though, it's more like "guarding the toll booth on the highway".
- Keyholes in Kingdom Hearts; you have to close them, but it's close enough.
- SCP-354, "Red Pool", from which monsters emerge regularly.
- Strange and horrible things regularly pop out of Jonathan Wojcik's The Fear Hole (bogleech.com) but the characters are a bit lax in guarding it.
- The claymation series The Trap Door had a Monster of the Week emerging from the titular trapdoor as its premise. And also an amazing theme tune.
- In The Life and Times of Juniper Lee, Juniper's home town is a hub of magical activity and her job is to protect normal humans and supernatural creatures from each other.
- The Ghost Portal on Danny Phantom. Which is a technological example, but aside from being uncloseable, apparently can't just be unplugged or something either.