I hope this doesn't sound too basic.....(for the people who say they are slow at modeling) do you all do some research online before you begin? That's VERY important. You cheat (simplify) where ever you can! If you can find a way to make that castle simply by finding what someone else has done previously, it saves you so much time trying to dream up how a castle looks.
Even though I've been a professional artist (in a previous career) and I can do just about anything art wise, I still have to cheat! I couldn't dream up a great looking castle off the top of my head either. It's soooo hard to re-invent the wheel every time you sit down to create something.
(Showing my age) Today, being able to do a google image search or a pinterest search is life changing!!!!!!!!!!!! In minutes you can see how dozens of people have already tackled the subject. Once upon a time I saved photographs from magazines of everything I thought I might ever need as references. My husband makes fun of that (and I still haven't gotten rid of many files) I have file upon file of cake photographs for instant reference. It's only been since pinterest began that I stopped saving and tearing apart magazines.
When you recreate items in 3D you need to think of the items as basic shapes and model them as such. Once you build your basic shape (a rectangle for the castle) your simply adding a couple cylinders (tootsie rolls) to each corner. The OP's design choice was PERFECT for her skill level! If she had chosen something more elaborate think of how many more hours she'd have spent on the cake. So when your looking for references pick designs that fit your own skill level. I could choose harder more time consuming designs myself and do them well, but WHY? Unless the client is willing to pay me for my skills, I don't have to give them the best thing I could ever do.....no I give them what they want to pay for and I adjust the design to that.