Good start, but my little bit of advice would be to think about the scale, an example would be to place a human model under the arch of the entrance then take that same model and place it up on a rampart. In this model I think the scale is off.
Even if the human is 1/4 the height of the arch he'll still look odd when placed on the ramparts.
a quick google image search of 'castles' should help you refine this model even more.
Like I said it's a good start, and it's definitely identifiable as a castle. I'm looking forward to see how you refine this model.
Cheers!