The ability to teleport to the abyss was possible because of the beam. But there was nothing within the Abyss to anchor their teleport spells to in order to get back to the overworld. The portal within the abyss might have been their answer -- another connection to another area of the abyss to bypass the distance needed to reach the rune altars. And because it seems like their magic was less refined than their Mage Guild counterparts, the portal could've been unstable, hence the Apprentice's need for constant vigilance. ...Or maybe I have it back asswards and it's the portal within the abyss that anchors them & allows them to teleport INTO the abyss, and the portal beam in Gielnor allows them to teleport OUT of the abyss as a reverse of what people use to teleport in the overworld. I'm not sure exactly which it is, but I'm fairly confident it's one of those answers. Actually I think it might be the second one. Ehh. I got a third theory: The power beam makes it possible to teleport both in and out of the abyss, but it cannot make the user's existence in the abyss stable. Teleporting in instantly forces you to teleport out at your destination. A normal teleport does not use the portal at all, so as to make the user spend practically no time in the abyss (which is a good thing if you want to avoid users being ripped to shreds). The portal is like the beam in that it's an anchor, but as you say, it's poorly executed and requires constant supervision, as the ZMI didn't have the resources or knowledge of the rest of the Tower to do it properly. Half-teleports, where the user goes into the abyss and stays there, uses the beam to go in and the portal to stabilise the user's existence in the abyss. Then this begs the question of how normal teleport spells work when you cast them while you're in the abyss. Ehh.