Use this as a moveevent on the tile below the teleport:
function onStepIn(cid, pos)
tp = {x=1663, y=2552, z=4} --the place where the portal takes you
bp = {x=1663, y=2552, z=4} --teleports back to somewhere close if not high enough
lvlreq = 10 --level required for
if isPlayer(cid) then...