Portals

Portal technology is not nearly as ubiquitous as most people assume. Portals are exceedingly useful if one has any reason at all to stay in communication with Earth, but otherwise there is no reason for a person to desire a portal. To that end, most people do not care one way or the other, and even those with family are usually content to rent a few minutes a month to send letters, or occasionally buy a ticket home and back. Also, they are well beyond the reach of the average consumer because they require a lot of personnel for maintenance, are a significant draw on any power grid, and have been known to randomly draw in such things as creatures of the abyss and demons when not properly calibrated.

Lots of Different Kinds

There are a wide variety of portal types, all of which can be easily imagined simply by looking up any particular science fiction of your choice. There are goo portals, magic stones, enveloping portals, portal shoes, magic circles, natural wormholes (also known as the Alice-in-Wonderland hole in the ground) and artificial wormholes. They can be constructed using spells, radioactive energy, an exceptionally large bonfire, electrical storms, asteroids crashing down, even the life-force of a virgin although this last is not recommended for myriad reasons.
Portals vary in strength and are usually graded on a scale from one to ten. One meaning the portal is probably unstable, so it can only be sustained for a few seconds, can only send one person in one direction, ends at a random location which may possibly be the top of an active volcano, and has a fifty-fifty chance of tearing the user limb from limb upon transfer. Ten is able to be active constantly, can send thousands of people back and forth, starts and ends in a chosen location and may be no more unpleasant than the sensation of leaving an air conditioned vehicle and going out into a humid summer’s day. In case there is any question, there has never been a level ten portal. Not that many have not tried to create it, but the stability issues are simply too profound.
Or rather, recreate it. Most of our historical evidence available to us indicates that each of the distinct races found in this world originally came from separate worlds. Most likely in the case of humankind one of them was Earth. These portals were, as suggested by the evidence available, stable two-way portals that facilitated early travel to this world. Whether this is an accurate view or not, these portals were either destroyed or not as stable as might be suggested. A few theories suggest the portals still exist somewhere but were simply buried on both ends. The truly outrageous suggest the aliens responsible for building the portals turned them off.
But in any case, theory or no theory, the result is the same. There are now only limited ways of connecting, and only to Earth. While it is vaguely possible that all the varied species we sport here originally came from Earth, it is far more likely that they all came from separate places, and for some unknown reason the portals we can generate will only go to and from Earth. This is likely the reason that humans outnumber the other races almost ten to one. Put together.

What it Feels Like

There is a wide variety of symptoms that portal travel can cause. The most common are nausea, dizziness and fainting, but these can also be attributed to panic and hysteria and may not necessarily accurately reflect true side-effects. Some portals cause hair loss, and other varieties cause twitching in the left foot for about a week. There was one rather infamous portal that gave the man who used it a split personality, but if you ever ask a portal technician about that particular incident they always insist it was a fluke and will never happen again. I believe many parents use the uncertainty to persuade their children not to go running off to Earth the first chance they get.
The sensations from traveling through a portal vary just like all the other variations. Those involving tunnels of some kind usually cause dry mouth, and the ones powered by electricity give a vaguely toasted sensation. The best kind are the goo portals, if one does not mind blurry vision for an hour or two. The sensation of being turned upside down and then back up again is the most universal description. In case it is not already obvious, those prone to nausea and motion sickness will not find their condition improved by a trip to this world.

Accuracy (or Lack Thereof)

Portals, at least as we understand them, do not need to be linked to a specific location. Whatever materials are used to open the portal may be easily picked up and moved to a more expedient location, provided the power source is equally available there. And make no mistake, portals require a great deal of power. This is why human sacrifice to open them went out of vogue, because it offers absolutely nothing to keep the portal open and it’s much more resource efficient to simply start out with an electrical storm and a very big red button.
What is more difficult is pinpointing where on Earth the portal will deposit travelers. A few unfortunate travelers have ended up inside trees or several thousand feet in the air, and these are among some of the least awkward possibilities. To further complicate matters, the end on certain types of portals has been known to jump locations, sometimes multiple times in one short stretch. With certain kinds of spells it is possible to end up close to certain destinations, although this is limited to the spell types and thus usually involve major landmarks of some kind. In case you’re wondering, it can be quite awkward when a minion suddenly appears on the top of Eirrat's Pyramid or in the middle of a secure art museum. Most people do not use spells if they can help it. The other option is scientific and requires precise coordinates, which can be difficult to obtain on occasion. By which I mean getting this wrong is how people wind up stuck in walls.
Thus, the vast majority of recent research has gone to the matter of safe, precise placement of exit portals. This has met with limited success and we are now able to at least deposit minions in the suburbs instead of the middle of crowded metropolises. Return portals are automatically drawn to the place they originated, which simplifies quite a few things. No minion ever reports difficulties getting back home, merely in the arrival on Earth.

Recent Usage

The vast majority of people to use portals are world leaders and their most trusted advisors. For obvious reasons, Earth is not a popular vacation spot and even those with family and former lives there are reticent to visit. The Overlord world is instinctively considered to be too strange, too far outside of the experience of those they would meet, and it is difficult to visit with old friends when one is forced to hide the best parts of your life. Certain supplies are gathered from Earth, for comfort and convenience, and there are of course kidnapping missions that many minions are employed for.
Especially in recent years, it has become vogue for Overlords to send their children and other types of heirs to school on Earth. The logic and grasp on more basic economic principles and diplomacy that is gained through this atypical education often proves invaluable when dealing with an average day in, say, Gorathia. To put it bluntly, we are simply too used to strange, and a dose of what is called “normalcy” is require to put things like budgets and galas and order forms in perspective.

Legend and Myth

So far as we know, the people of Earth remain largely unaware of the existence of the Overlord world. Those we kidnap are partially selected because analysis indicates they would live average, invisible lives on Earth and so are not missed. We take great pains to avoid being noticed by Earth’s leaders, because history and a wide variety of entertainment choices indicate that they would not take it well. Also, on the off chance they would in fact open a dialog it would invariably end in someone or another trying to conquer them, and Earth has enough problems without adding the unique political cycles of the Overlord world.
There are a few slip-ups, of course. Legends and myths are born when Tommy from accounting encounters a cascade tripwire and lands in Stonehenge under a purple cloud. We are, perhaps, fortunate that the truth is too implausible.
There have been a few individuals who believed that Earth should be theirs for the taking who endeavored to cross over and take control. These efforts were quickly and decisively dealt with by one of the only global alliances in history. Since then portals have, as a whole, been deemed as dangerous for Earth just as much as for those who live here in the Overlord world. At the very least, any person attempting to conquer Earth now understands that complete control of this world would be necessary first.

A Bad Idea

As a quick addendum, since information has come to light about certain experiments scientists of the Castle of Terror been conducting, entropy portals are bad. Deliberately causing a situation that randomizes elements of an already unpredictable and chaotic process is a foolish endeavor, and invariably ends in some variety of monster going on a rampage through the laboratory. Or, as the case may be, several monsters.