One way. At first, Isabelle thinks the girl means that it's one way home, and she's ready to go dive back in head-first if that's what it takes. But then she realizes — it's a one-way portal. You pop out of it, but you can't go back in. If there's warlocks around, they should be able to open a different one, or reverse the one in the fountain, or something.

Unless something's happened to their magic. Isabelle's noticed (and tried her hardest to ignore) that she hasn't felt quite as strong since coming out of the fountain. If anything, the adrenaline the situation has brought should make her stronger. But it hasn't.

"How do you accidentally leave somewhere?"