You had me at first-person castle sieges and warhorses. Warhorse Studios has started Kickstarter funding for the open world, action-adventure RPG Kingdom Come: Delivernce to “fuel ongoing investment in the game.” If successful, it will allow Warhorse “to create additional content and a better overall product.” Kingdom Come: Deliverance is scheduled for a 2015 release on PC and next-gen consoles.
See the Kickstarter pitch below as well as screenshots of the game in action.