"Truthfully, I don't think he built it in any of these places," Twilight said tiredly. "Celestia's own research to find the retreat all came up with the same ideas as us, and they're all silly places to build his secret retreat. What she does mention though, was that he managed to research these places, and be of service to Equestria, all at the same time, and looking at some of the dates on these books, I have to agree. He'd finished writing a paper on the indigenous creatures of the Badlands two days before he started one on the significance of the frescoes in the temples in the Tenochtitlan basin. Then a week later he started researching something else in a different place before he'd properly finished that, all while being at Celestia's beck and call. How could he have possibly gotten around that fast, and still be available to Celestia?"