Last Exile/WMG

Prester is not a planet.
If you think about it, a planet shaped like an hourglass makes no sense. How are planets formed? By cosmic space dust that condenses into a large mass via gravity. Realistically, there is no way Prester could naturally have formed that way, and, shaping a planet doesn't seem physically plausible.

However, being hourglass shaped DOES make sense if you think about a massive space colony, which would need to generate Artificial Gravity for the population by spinning to produce centrifugal force. Which would in turn, help to explain how the Vanships can reach the necessary height to enter the Grand Stream without using lift aerodynamic principles, as a space colony would have increasingly less gravity as one heads towards the center.

In addition, it wouldn't make sense for a planet to have two extremes of environment, one suffering tundra like freezing while the other side almost having a drought. This is actually Lampshaded in the anime, where the characters point out that the guild has been neglecting its duties to keep Prester in order. If the guild hasn't been maintaining the Colony, then its environmental controls could have deteriorated or even malfunctioned. Which would explain why Diseth is drowning in ice, but Anatoray has barely enough water to survive, its the result of computer error rather then weird natural phenomena.
 * That's pretty much confirmed by Word of God. Prester was one of many habitats created out of asteroids for humanity to inhabit until the Earth's environment became stable again. The Guild was supposed to keep Prester in working order and transport the colonists back to Earth after a couple hundred or so, or if something went wrong. It's all written in Official Artbook and it's implied that there are other "Presters" where the populace ended up wiping themselves out.