This is a good example of making instances with a level requirement if you want it to scale with lets say levels you would make multiple versions of the zone and change the script to reflect the version you want it to send them to and the level required to enter.
http://www.eqemulator.org/forums/showthread.php?t=41476
Also the link at the bottom of the linked post above references Akka's post on the same subject which is much more advanced.