Well, mainly just the practicality of this castle. I'm willing to shrug this castle off simply for artistic license.
But looking at it from a practical side, one has to wonder what this castle's purpose is, as in its tactical benefits for its location in relation to its surroundings.
There's also the question as to whether it really needs to be a castle at all, as siege weapons can't reach it, there's no good place to shoot the defenders from, and so the only way to siege this castle would be to camp outside its gates, and maintain pressure at the postern. Indeed, simply a manour would suffice.
What kind of enemies is the castle there to defend against?
There's also certain battlements that don't really need to be there, like the crenellations and the machicolations. In fact, most of the castle wall could be left out and its defensive value would be more or less the same. Due to its location, it is in the perfect, most defensible location you could build a castle.
It also seems large for a castle, doesn't it? Castles historically were deceptively small, only consisting of a few rooms, really. If you look at the top-down layout of most historical castles, it becomes clear that they're actually quite simple.[Layout of Conwy Castle in Wales][Layout of Windsor Castle in England][Layout of Harlech Castle in Wales]
Of course, none of this is meant to be me saying that I don't like this art; on the contrary I like it a lot. It's simply the part of me that loves castles and castle design that is speaking here.