Yes but where.

If he looks for pictures of it he'll see a stone archway with mystical symbols engraved on it framing shimmering blue-green-purple light standing atop a circular slab of stone with more arcane etchings in the middle of the river. From this vantage point it looks like there's a waterfall right beyond the portal.

Okay, waterfall means that he should probably follow the river?

There's a dirt path lit by streetlamps that continues along it from where Peter is, and it doesn't take much walking for him to go around a bend that brings the portal to view. To get to the portal he will need to walk to the edge of the river and then skip over a little bit of water towards the stone slab.

...skip over water. As in, river water?

Yup.

As in if he slips and falls he might be dragged towards the alarmingly tall waterfall to fall to his untimely demise amidst the rocks at the bottom?

That's right!

Who. The fuck. Designed this.

But fine, YOLO, worst-case he becomes a ghost.

He successfully reaches the portal!

Yay! He is not dying today!

What's this portal like, then?

It looks like the picture. The shimmering light wavers as if influenced by unfelt wind, and from up close it emits a low, soft hum.

So does he just... step through?

Only one way to find out!

He supposes so. Here goes nothing.

It feels almost like how water felt on his skin when he was a ghost, except for how it doesn't feel like that at all, and what awaits him on the other side is an ordinary if rather large house and extraordinary surroundings.

The sky is purple and pink with streaks of blue, and it's everywhere, because he is clearly no longer standing on firm ground on a planet. From this vantage point he might assume that he's standing atop a very tall plateau but that impression is ruined by something that looks like a small floating rock island in the distance behind him and to his left, not to mention the floating rocks swirling around it. It doesn't take much walking to confirm that he is also on one such island, himself, and if he was scared by the waterfall we've got bad news for him about the existence of a bottom to the particular abyss he'll find if he peers past the edge.

Oh. Okay. He will. Carefully take some steps back, lowering his body close to the ground because his legs are going uncomfortably wobbly in what is probably the least helpful possible response his body could have to the vertigo of an endless void.

But cool. That's cool. Floating islands. Very aesthetic. Dangerous, terrifying, but aesthetic. He'll just sit here for a moment, though, thank you.

No one comes to bother him, although he can see a couple of people flying by on flying broomsticks.

He supposes flying broomsticks were to be expected of witches, really. And are those the only way to get to the other islands? Those do seem to have some buildings on them so presumably people go there.

No, if he follows the cobblestone roads he'll find portals very similar to the one he came through near the edges of his island that probably lead to the island that can be seen beyond them.

Or they might drop him into the void! Who knows. Not him.

Alright, enough gawping at yet more and exciting ways to die, if he was dropped right in front of a mansion presumably it's an important mansion, yes?

Presumably!

The ground floor is dominated by a large, mostly-empty square atrium. There are steps leading down to a depressed center where a slab of stone identical to but much better-maintained than the one under the portal set into the floor, runes glowing brightly. You can look down into that room from the first floor's mezzanine gallery and there are arches leading into other rooms on all three other walls.

...magical dueling room, maybe? Looks like people can spectate on stuff and it does look fancy.

But how about people, are there people?

Not in this room, but he can hear faint murmurs suggesting the other rooms might be occupied.