This is epic, man! You've got knights charging, dragons, castles, awesome, rich fantasy scenery and awesome lighting effects to top it all off. It's kinda like Gears if War. Ya just sit down, ask yourself what's all the awesome stuff in the world, then throw it all together. Jeeze.
Honestly, there's only one point I can think of to critique, and that's that the castle looks a bit strange without any windows or texture or anything on the sides in a perfect square. The shape and everything is cool and unique, but it kinda brings out how plain the sides are which is also juxtaposed by the well textured and loose (an awesome combo) rest of the painting.
Wait, your first time painting horses? My word, if this is a first time on that you're one heck of an artist. I can't even sketch something that looks remotely like a horse, let along a lot of really good horses in an additionally awesome picture. Great piece, it was a pleasure to stumble over this piece.