The zip contains 2 RMFs and 2 BSPs. The maps show the simplest of examples of using trigger_changelevel entities and info_landmarks.
Run "bj_example1.bsp." Go through the portal and the map changes to "bj_example2." Go back through the portal to return to bj_example1.
In each of the two maps, the portal is a brush textured with the AAATRIGGER texture. The brush is tied to a trigger_changelevel entity. The New Map property of the trigger_changelevel entity is set to the name of the "other" map - in bj_example1, it's set to "bj_example2" and, in bj_example2, it's set to "bj_example1."
The Landmark Name in BOTH trigger_changelevel entities is set to "cl1."
The portal is surrounded by simple world brushes to create a "doorway."
An info_landmark named "cl1" is placed in each map about where you want the player to end up when he goes through the doorway and changes maps.
NOTE: The info_landmarks and the trigger_changelevel brushes don't have to be in the same location in the maps. One set of entities can be on the west side of one map and on the east side of the next map. But the naming conventions described above MUST be adhered to - each trigger must point to the next map and BOTH info_landmarks MUST be named the same to go back and forth between the maps.