The manual for DKC3 states that this game only takes place several months after DKC2. Building a fancy castle like that in a few months seems impossible, especially since K. Rool lost a huge part of his Kremling army after Crocodile Isle sunk in DKC2. He may have gathered all Kobbles, Krumples, Re-Koils and all of the other DKC3 Kremlings and made them build the castle, with himself just being the architect who designed the castle, but even that seems unlikely. To debate that theory, we'd need to discuss what the Kremlings from DKC3 even are. They certainly look different from the Kremlings we saw in the first two games, but does that mean that the "K. Rool experimented on them" rumors are true or does that mean they're native to the Northern Kremisphere? I believe that the former is true. The game talks about KAOS like he's the new boss of the Kremlings, but at the end of the game you find out it's actually K. Rool who built KAOS. This means that the Kremlings are under K. Rool's command, and I don't believe some random Kremlings would just obey a self-proclaimed king of the Kremlings who lost everything while trying to beat DK twice. So, that leaves us with the question "Are the Kremisphere Kremlings capable of building a castle within a few months?". I think so. But that's kind of a boring theory if you ask me.
Baron K. Roolenstein also has ties to magic. He sealed the Banana Bird Queen away with a magic barrier, how can that be explained by science? Not to mention the eerie purple glow coming from the castle's windows. The questions is: What kind of spell just conjures a castle? Creating a barrier to trap someone seems easy enough to pull off for a rookie magician but summoning a huge castle that's perfectly suited for you? That doesn't seem plausible at all. Still, I think magic definitely played a role when it came to building this castle.
Of course, you could also say that the castle was there before K. Rool ever got to the Northern Kremisphere. Although this theory makes the most sense, it still doesn't sit right with me because of how perfect the castle seems for K. Rool. I mean, just take a look at this room:
That little rounded wall where the curtain is, the scientific equipment, the usage of the color green. It all seems perfect for a Kremling inventor. However, he might have become an inventor because he stumbled across these machines inside the castle. Maybe the books from that bookcase on the right hold the answers to all of the technology there...