Create blockouts for the various buildings and other main structures you want to include in your environment. These should be very simple groupings of shapes, without worrying about the details or textures. 
 

Choose a single small building from your blockouts and focus on adding details to just that one structure. Keep in mind the general scale of the environment you might want to create. If this building will be small in the final shot, you won't need many fine details. Create a high-res and low-res version of your building for easier use in Blender later.
 

Now that you've created your first building, it's time to detail out the rest of your small and mid-size buildings or structures. Essentially, in this stage you want to work on everything except for the 'hero structure' of your scene (in Pablo's case, this would be his castle). Make sure to create low-res and high-res versions of each structure. Try making variations on each finished design as well, to generate even more diversity in the look of your environment. 
 

Now it's time to work on your main building or structure. This would be whichever will be the centerpiece of your environment. Take as much time as you need here, as this building will likely include more detail than any of the others you've done previously. As with the others, create a low-res version of this building in addition to the high-res version.