Guardians are honestly one of my favorite monster types. They represent a mixed possibility of social or physical interaction- you know, depending on the creature.
Different guardians can be approached different ways. Every guardian-type monster should have something that they want to hear, something they want to see, or some sort of bribe they'll accept. If you can give them that, or convince them to accept something else, you're set. Of course, sneaking in might work. Or a group could try a persona...
Regardless, here's a basic guardian-type monster.
An undead servant that slays any living creature that attempts to pass. Usually well hidden, as the presence of guardians indicates the presence of treasure to watching eyes. Skeletons want to intimidate enemies into surrendering so the skeletons can kill them, because the only thing skeletons hate more than guarding places for all eternity is life itself.
0M, 100B, 100S, 10R
Strike Bonus: +2
Rusty Junk- melee, 10M damage, 5B damage.
Eternal Approach: A skeleton takes no damage from falling, and can stack vertically with other skeletons, forming a ladder of skeletons.
Skeletons deal some damage directly to Bone, making them dangerous to fight for long.
These skeletons are the hollow-voiced nightmare guardians that one could find in fantasy media in the 80s. They are armed with some sort of weaponry, but their main threat is the Bone damage. It's not much, but it can be enough to make further progress into the site they're guarding unwise. They can also stack on top of one another and be climbed, so terrain matters less to skeletons than other creatures.
Here's another guardian, a classic:
Sphinxes guard tombs and treasure-houses. They are not actually living- rather they are a magical construct that does not need to eat, sleep, breathe, or see. They demand a countersign as the price for entrance, and violently slay any who attempt to enter without giving one. Common countersigns include the answers to riddles, a nonsense phrase, or possession of a singular object.
200M, 200B, 200S, 200R
Sphinx Claw- melee, +7SB, 40 M damage
Command- ranged, +9SB, 30S damage.
Sphinxes can fly and can see every location in the encounter at all times. A sphinx can speak all languages. A sphinx's sole desire is to prevent visitors. A Sphinx can use ranged powers even when a hostile creature is adjacent to it.
The classic sphinx demanded the answer to a riddle, or it would eat you. This Sphinx provides the same. You can't sneak past it, and it doesn't have any particular desire to leave at any point. Since they are impossible to dislodge and don't age, they are often very old, and may have been set to task centuries ago. Intruders who cannot answer correctly are commanded to leave, and intruders who cannot speak are simply torn to bits.
There are a couple more guardians I have in mind: The basilisk, the gorgon, hydras, minotaurs, sirens. The focal point is that characters have a hard time getting past them, and, of course, they are in the way somehow. In a GM's toolbox, Guardians are a known threat that asks players to think a little outside of the box. Encountering them head-on is often foolhardy and they absolutely won't get out of the way, so they require a bit of lateral thinking or else the expenditure of resources. I like to use guardian type encounters in most games as a sort of final check, so that players can do what they like once they gain access to the location, but getting what they want requires some hard decisions. Hard decisions are the best part of roleplaying games, so put some thought into what guardian to use and where to put it, and always consider that the players will come up with a better answer than you anticipated.