The Weather Problem.

"You aren't going to do any work?" Aina sent a glance back toward Leonel, standing over a beast corpse. 

As opposed to Earth which was mostly water, the world Valiant Heart Mountain resided on was mostly land. This not only made water an extremely valuable resource, but it also made controlling ownership of land just as difficult. 

One would think that the reason Valiant Heart Mountain was set up at its current location was because of the seven Peaks acting as a great protective barrier. But, the truth was that this was only a secondary condition. The main reason that location was chosen was due to the floating islands. Namely, the waterfall phenomenon on said floating islands. 

With the Valiant Heart World being over 90% land, and with the remaining 10% of water being mostly salt, it was no surprise that such freshwater sources were incredibly valuable.