I have recently started modeling a Castle in Google Sketchup. Here's what it looks like so far.
Here's the drawbridge mechanism. It's a little messy because I was in a hurry and have no extensions.
I also added a balista.
There's also a little box for archers to shoot from.
And of course the front gates.
It has a fountain.
And some sort of building.
Last but not least, the barracks.
Hope you enjoyed, and leave any suggestions in the comments.