I'm pretty sure it came up in the Intermission, but it wasn't really "Jack built the green city". It was just Jack saying to himself "I [you] built this town", as Made Men love to say to themselves. There was a hint that Jack was one of the "Founding Exiles" on Alternia much like WV on Earth, but I do find it kind of hard to believe that even with three henchmen he would swing a hammer long enough to build a fake New York.
He has to have had a serious hand in building it, because when Dies (I think) put his pin in the doll, the city disappeared. Since Dies'(?) power moves him to a timeline where the target of his pin is dead, but not forward or backward in time, we must assume that Slick had a serious role in making the city. Or is otherwise so intricately tied to the city that he cannot be dead in a timeline, at that particular moment, where the city existed as it did then.
I wasn't arguing the fact that Slick built the city. I know where it says that he did. It's in the first Intermission, near the beginning of the whole ordeal if I recall correctly. I was questioning his motivation for building the city.
Oh, and the green city is not the city Jack built, because Jack was on Alternia, and the green city is on Alternia's moon. The green one.
Was this ever actually stated anywhere? I certainly don't recall it. I don't have an eidetic memory or anything, but I've reread Homestuck ten or twenty times, so I'm pretty certain it was never said there.
I don't recall it either. As far as I can remember, it's never been stated what Spades Slick did in his city or to its inhabitants. You can make assumptions, because of crime lords and prohibition theming, but never anything set in stone.
There was mention in the Doc Scratch marathonologue that if HIC looked closer at the surface of Alternia when she arrived, she would find a city of dead exiles, but I don't take that to mean Spades Slick built the city to lure people in and kill them.
If we take all events in the Intermission as canon, which I think we're supposed to, the city is full of dead exiles because Slick forced English's safe open with Crowbar's crowbar, which unmade the safe and did some seriously bad shit to everything. Of course, I find myself wondering how, precisely, Slick made it to the moon.
Shenanigans, clearly.