Camelot Unchained team that this game is meant to be large. The focus is on large-scale sieges, not a couple of guys banging on the front door of a castle. That means the game needs to be able to handle a huge number of players on the screen at any one time. How do you find an engine that can handle that? According to the latest update on Kickstarter, you build one.
While the engine isn't built entirely from scratch, it isn't based off of an established engine, simply because it's being custom-built to handle the game's specific needs. The attached video allows you to see the engine in action, scaling from a handful of characters up to the moving mob pictured in the header. If you want to know more about what makes the game pretty to look at, this will be very relevant to your interests.