"How different should I be expecting the magic of another world to be? Will it work on different principles? Use different resources?"

"Likely. Some form of "mana" is common to many worlds, but some have magic systems that work on completely different principles. Magic is broadly recognizable as magic, but that's all I can say."

"You still expect your magic to work here; should I expect the same if I visit your world? Can you cast new spells and improvise new tricks here, or only use tools you've already created?" They turn down another lane and head toward the Tower proper, entering it by a small side door and starting up a narrow spiral stair.

"The magic that I know I can use here is in my tattoo, which makes it part of me and not this world. I could try some other magic, but that is not always a wise idea. Some worlds have nasty reactions to unapproved spellcasting, which might even mean any spellcasting at all. OTC maintains neutral casting environments where most mages can practice. My native world, from before I became an employee of OTC, would probably let you cast."

One turn of the tightly winding stair takes them past an arrow slit with a view of the citadel from right above the door. At the next window, they're high above the ground, looking out over the rolling hills. At the next, they're a little lower down and looking out over the water. Senafia keeps climbing.

"I'd love to travel all the worlds and learn their magic," she says. "How hard is it to move between them?"

"Once a link is established, usually not very difficult. It's doing the initial establishing that's my job." 

The next few turns of the stair have no window at all.

"What governs how hard it is to move between worlds? You mentioned different worlds having different opinions about foreign magic—I assume that's metaphorical opinions—I shouldn't assume that, there may very well be worlds that have the regular kind—but is there something that has opinions about the whole business? Are there things that hold true everywhere? —is that form natural to you or did you take it to fit in, do the people of different worlds all differ from each other as much as humans differ from birds or elephants?"

"I look like a human because of complex reasons, most important of which are "this form expresses me well" and "this form is meant to blend well into any random universe." So it's both natural to me and a blending measure. Intelligent nonhuman species exist: humans are multiversally common, and humanoids more common than that. Most places are recognizable as worlds, or at least, most contactable places are. It's easier to search for similarity to existing things. There are at least three worlds I can think of that have opinions of the regular kind on unauthorized spellcasting, and at least a dozen that are metaphorical opinions. Mechanical motion is reasonably reliable. Some form of gravity - the thing that makes things fall - usually exists. This is because we search for planes where the physical laws are similar enough to our native plane for the world to sustain non-utterly alien life. So far as we can tell, most universes are uninhabitable on the level of "you would explode if you tried to enter it." So we take care to index the ones which have anything of value in them."

"Why are humans common? Is it just that you keep looking for places that are like the places you know, or is there something else going on, or are you not sure? —and here we are," she says, opening a door.

The room on the other side is spacious, with a vaulted stone ceiling high overhead and a well-swept stone floor holding neat, sparse rows of desks and tables, each with its own heap of scrolls or row of beakers. All four walls are lined with bookshelves except where the shelves are interrupted by a door.

"Welcome to Research." She makes a brief gesture as though plucking the string of an invisible harp, and addresses the empty room, "This is Thorn, everyone, she says she's from another world and she wants to open a door there to prove it. She was talking to Treasury but Treasury sent her here because they love us and want us to be happy."

A flurry of doors pop open all around the room. Excited women pour through them.

"Who's got a workspace they can spare?"

"Oh, use mine," says the youngest one in the room, bouncing on her toes. "I haven't started anything yet today, I've been catching up on my reading."

"Sure," Senafia says agreeably.

"I'm Esryne," the girl adds, beckoning Thorn over to her door. "It's lovely to meet you!"

"We're not certain why humans are so multiversally common," she says. "But that doesn't mean we can't act on it." 

She smiles at Esryne. "It's lovely to meet you too! I see there's a lot of you here in Research."

"That's because Research is the best, obviously!"

She laughs, and heads inside.