On a world so wreathed in magic, it is perhaps not surprising that the veil between worlds is somewhat... thinner in places than it should be.
Portals
In such places it is possible for a portal between worlds, or simply between two places on the Mortal Plane, to be created with the right know how. Often they are marked, in particular gates from one part of the mortal plane to another are signalled by certain types of stone circles with a cromlek in the middle (an arch made of 3 large stones).
Gates
Portals must be specifically opened to be used, using rites and rituals to ignite their latent magics and to combine the elements in order to open them. However, there are certain places on Tymaera where portals are permanently open. These are called Gates - there are nine in total, ergo, the Nine Gates.
The most infamous of these is the dreaded Dreaming Gate far south of the World's End mountains. This gate allows energy to leach through from the Chaos Realm and is where the godless chaos demons come from.