How does Repulsion work?

[Seems]

First time adjudicating this spell, so I want to get it right. The spell reads:

"An invisible, mobile field surrounds you and prevents creatures from approaching you. You decide how big the field is at the time of casting (to the limit your level allows). Any creature within or entering the field must attempt a save. If it fails, it becomes unable to move toward you for the duration of the spell. Repelled creatures’ actions are not otherwise restricted. They can fight other creatures and can cast spells and attack you with ranged weapons. If you move closer to an affected creature, nothing happens. The creature is not forced back. The creature is free to make melee attacks against you if you come within reach. If a repelled creature moves away from you and then tries to turn back toward you, it cannot move any closer if it is still within the spell’s area."

So, what constitutes "move toward you"? Can an opponent who fails the save move towards an ally of the spellcaster standing next to the spellcaster, or who is standing 30' away from the spellcaster but still within the field of the spell? Or is the opponent obliged to stay outside the field of the spell (unless the spellcaster moves closer)?

I'm guessing the easiest way to adjudicate it is to say the opponent has to stay at least as far away from the spellcaster as when they failed the save. In other words, if the opponent was 30' away when they failed the save, they can't move to any square that brings them closer than 30' away. This way they could run around the spellcaster but not actually move towards them.

[Kayerloth]

Correct, the presence or absence of teammates isn't taken into account. If in your example the foe moved 10ft further away but remained within the field they have effectively set the new minimum to 40ft as well. They could move around in any direction including flying, swimming or burrowing as long as the don't get closer to the caster. Only the caster can change the minimum by moving closer to his foe(s).

[Dave Justus]

The can't move closer to you.

If the square they want to go to is closer to you than the square they are currently in, they can't go there.

You are making it way more complicated than it is.

[MrCharisma]

Seems wrote:

I'm guessing the easiest way to adjudicate it is to say the opponent has to stay at least as far away from the spellcaster as when they failed the save. In other words, if the opponent was 30' away when they failed the save, they can't move to any square that brings them closer than 30' away. This way they could run around the spellcaster but not actually move towards them.

+1