Gateways and Realm-gates

Now you're thinking with Portals!

Gateways

Gateways are large constructions of various materials and make that bend space, allowing a connection between two of these constructs.
They allow for the rapid exchange of goods or people over tremendous distances.
Gateways can connect any two points within a Mortal Realm seamlessly and instantly.

Costs
Gateways are very difficult and expensive to build and maintain. Each Gateway needs a tremendous source of magical power to draw on to operate and trained specialists to control and maintain it. The Operators also need to be highly trustworthy.
This requires a highly sophisticated understanding of magic. Indeed, teleportation and the building of Gateways are one of the highest disciplines of magical research. Needless to say, this is also very expensive.
For this reasons, gateways are only ever constructed in larger numbers once the civilisation gets very magically advanced and wealthy.

Risks
Gateways can be easily hijacked by enemies. While it is possible to create blocks to large-scale teleportation, these cannot be placed around gateways intended for active use as they would stop working. This means that Gateways are open to becoming invasion vectors from outside. Even undiscovered nexus points may be an invasiom vector if not monitored or sealed.
For this reason, gateways are often highly guarded, set to self-destruct at a moment's notice, as well as set in appropriately chosen locations.

Realm-Gates
A Realm-Gate is a more powerful and even more complicated variant of a Gateway. They can only be constructed in special locations called "nexus points".
A gateway can only connect to another of its kind on the same mortal world. However Realm-gates allow travel between one or more nearby mortal worlds.
It is by Realmgates are the main way that advanced civilizations connect multiple worlds into a singular empire. If a suitable nexus point is discovered, divination magics and summoned spirits are used to determine the nature of the world on the other side. If it is considered worthwhile a team of colonists are sent through and Realmgates are constructed on both sides, finally allowing the passage to be fully opened and the world is colonized.

Nexus points
There exist special locations on any mortal realm called 'Nexus points'. A Nexus point connects to one or more nexus point on other realms and these connections can be opened with the use of magic to create a passage to travel through.
Nexus points can appear anywhere on a mortal world, including inside of terrain, under the ocean or in mid-air. They are an invisible convergence of energies and are only detectable by those capable of using magic and even then only if one is actively looking for them.
To complicate matters further, Nexus points can appear suddenly and disappear again just as quickly, lasting only for a few days or many centuries based on the movement of the mortal worlds.

Notable variants

Humanity does not possess the capability of building Gateways. While allegedly they used to be able to build them before the War of Burning Skies, the knowledge is now hoarded by a precious few if it even exists anymore. Some relics remain, though they are jealously guarded.

The Sidhe prefer to keep their worlds isolated and self-sustaining, with limited trade. As such they do not maintain many gateways and only one Realmgate per world, apart from on their home-world of Avelon, which is connecting their realms together.

The Dvuergr maintain rather more open portals than any other civilization, often having one in every city of even moderate size to engage in commerce. Notably they are very paranoid about security and tend to line their permanent Gateways with explosives.

The Khuzu dont use permanent gateways. They technically have the ability to construct them but they barely engage in trade with each-other and don't trust the other Khuzu city-states not to immediately hijack an open gateway in order to invade and raid them.

The Jotnar are desperately trying to maintain their network of gateways, but they have forgotten how to build new ones and only somewhat understand how to maintain the ones they still have. Every year more of their constructions fail.

Teleport Anchors

In the Brass Realms, teleportation is possible. Be it of a single individual, a small group or an entire army, all of them are possible, if not equally easy.
So, the societies of the various civilizations across the realms have developed countermeasures as a means to survive.

These countermeasures to teleportation are to create a magical barrier akin to a stone wall, to surround a defensive position and keep out intruders.
In order to erect such a barrier, a series of "anchors" are placed throughout a location, such as a fortress or city. These locations are then protected from outsiders teleporting inside. Very often these barriers still allow teleportation from the inside out and from the inside to inside, but may also block all teleportation completely.
Inside a protected location there is often secret portal locations with defensive emplacements surrounding them, deliberately not covered by the barrier in order to allow for outside reinforcements or evacuation.
The locations of such transport points are a highly guarded secret.

Weaknesses
Teleportation barriers can be forcefully breached by a deliberate magical attack, but it is exponentially easier to simply sabotage the anchors that maintain the barrier. Thus, sabotage and sieges are still the best way of conquering a defensive position in the Brass Realms.
Teleportation barriers are noticeable to expert mages, and are detectable with a variety of magical instruments.

Notable variants

Humanity possesses the ability to create teleport anchors, though they are usually only active in critical locations or during times of war, as their continued operation is too expensive. Their Teleport Anchors are made of metal and Riftstone.

The Sidhe have developed a special kind of tree that fulfills the same purpose as a Teleport anchor. The trees are very cheap compared to building a teleport anchor but take a very long time to grow. However due to how spread out the sidhe usually live, they don't actually have many critical locations that would require one of these trees.

The Dvuergr line all their cities in constantly active warding rune-stones as a matter of course. They managed to refine their expertise of these devices a lot in order to make them both more sturdy and cheaper to maintain. However they are enormous and very expensive to produce in the first place.

The Khuzu keep their cities equipped with Teleport anchors at all times but often lack the resources to fuel them constantly. Thus they rely on a strategy of turning on their anchors only if they are under attack, cutting off reinforcements while they turn their city into a trap for the invaders. This strategy is only partially effective.

The Jotnar have almost no teleport anchors left. They guard their treasuries and the bedchambers of their Kings, but their cities and fortresses are largely exposed.

The Tu'vok are particularly adept at producing gateways formed out of purpose-shaped portal-organisms. They also spread jammer-organisms all over their domiciles, rightly paranoid about their enemies.

Appearance:

Gateways can take a variety of forms. From permanently active portals to paired platforms that exchange their contents when activated and more.

The specifics about their construction also vary greatly. The Tu'vok create living archways of meat and Chitin while the Jotnar might use elemental ice or stone.

Realm-gates are built to open passages between worlds rather than inside worlds. Realm-gates are usually much larger and more expensive to create.