When you are building a room, there are only a few pieces to prepare. Before starting to build your set, map out the parts you need.
A complex set that is more intricate in its design will require more careful planning. Not only will you have to list the pieces you need, but you will also have to think about what can be reused. If you have a series of windows in your set, you would create a symbol from the drawing, rather than copy paste it several times all over your set. This way, if you need to make any correction to its design, you will only need to do it once and all the instances of your symbol throughout your set will be corrected. The same can be done with for each piece of your set.
Here is an example of all the source drawings required to build the castle.
These are the only drawings required to build the entire castle. As you can see, there are not that many considering the final result. The rest is constructed by assembling symbols and through reuse. However, keep in mind that although you are using simple drawings and symbols to optimize your set, the more you add to your project, the heavier the toll it takes on your machine's RAM. While the scene's package itself is really light, running the scene places heavy demands on your RAM and video card.
Using the drawings shown previously, here are a couple of pieces that were put together.
Here is the final result. A large and impressive fortress.