Object Design: Castle

I am learning how to use a program known as Rhino 3D.
To help with learning the basics, I made a castle using this program.

I started by making some basic shapes, essentially drawing a blueprint of the castle.
I used circles, squares, rectangles, and a single point to make these shapes. To make the squares perfect squares, I pressed shift, which locks the shape so it can only be a perfect square.

It has four towers, an outer wall, and an inner courtyard. The circle to the left is for the tops of the towers. I used osnap to align everything, it's a very handy tool for making sure that things are exactly where you want them.

Next, I changed to perspective mode and made the shapes 3D.

Once I had done that, I hollowed out the insides to make corridors, and so that the towers were also hollow. Not a very exciting castle if you can't go inside. I made smaller shapes, made them 3D, and connected them using the command Boolean Union, which makes the shapes into a single unit.

It was a bit tricky trying to make sure that the hollow part of the towers was connected to the corridors. I had to play around with a few different shapes to get the correct width. I used a function called Boolean Difference to subtract the one shape from the other.

I had to add another shape, a square with a half circle on top; I almost forgot the door!

I completed the door, made it 3D, and subtracted it from the castle walls. I first just had the opening through only the outermost wall, but thought it looked more authentic with it going through straight into the courtyard.