That's Castlelands! I built those a long time ago on singleplayer, before I started playing online, and yes - the server was down for a while so I could MC Edit some stuff to non-snowy biomes. It was giving me a hard time with reverting the previous area back to pre-build status, so I just ported in the castles.
The smallest of the castles is 100% legit - everything in it was built entirely by hand and no cheats were ever used to obtain the goods. The gigantic prison-like one was laid out by hand entirely, then I discovered Inv Edit and started using that to do the building. A small amount of it used MC Edit to build, but only a little. The last one - the one with the castle town - was built almost entirely with Inv Edit infinite resources, but no MC Edit.
Problem is, the one castle with the town inside the walls went super buggy - I have no clue why or how - and there are a couple hundred minecarts just chilling there, buggin' out. I spent some time yesterday doing some heavy demolition on them, but I only managed to thin it out far enough that it didn't cause enormous lag for me. There's still a tonnn of them there. Cleaning up after that and repairing the damage caused in the cleanup process looks like it's going to be my primary job on the server for the weekend, at least as soon as I finish connecting my city's new minecart subway to spawn.