In the cosmology of the World of Mythandar©, all of Creation was once divided into seven planar realms - the elemental domains of Air, Earth, Fire and Water, the Realm of Light and the Realm of Darkness, with all of these bordered by a dimensionless Realm of Shadows.
As the learned know, the universe is constantly in motion, and so these Seven Realms revolved around each other in multi-dimensional fashion. Occasionally, they would come into contact, their collision manifested as a great surge of Power which would open a rift between worlds. These Gates were sporadic and unpredictable, lasting for moments or millennia, but always closing again as Creation continued inexorably on ...
Then, at some time in the distant past, the gods caused the four elemental realms to collide, merging into a single "reality" and reducing the multi-verse from seven realms to four. Now, with Creation more ordered, Gates still occur, though they are rare and more stable than before. A Gate might remain in place for centuries or millenia.
Currently, there are five known "permanent" portals in the world. Other, lesser gates exist, connected to each other and with the great primordial Gates. These portals are stationary (from our perspective), remaining in certain locations and accessible to those with the ability to detect and open them.
Not all Gates are known, and the location of a functioning portal in Mythandar is a rare and valuable piece of lore. All minor gates are linked together and can be traveled between. In varying sequences, they are also connected to the four so-called Elemental Gates scattered throughout Mythandar. Those are themselves connected to the central Great Gate, located at the Heart of the World and from which (it is believed) a person might be able to travel from this reality to another Creation all together.
In other words, there is only one way to enter and leave THIS multiverse - through at least one minor gate, one of the four Elemental Gates, and the Great Gate at the Heart of the World.
That should keep X-Men, Masqueraders and Ninja Turtles out of Mythandar.