A lot of times, opening a 3D modeling software package and building architecture that way is what professional games studios will often do. This gives you maximum freedom in creating your environment and allows you to do exactly what you'd like to do. However, this requires you to be proficient in that tool, whether that be Maya, 3Ds Max, Blender (which can be downloaded for free at blender.org), or some other tool. Then, you just need to export your models and import them into Unity.

Unity supports a lot of different formats for 3D models, but there are a lot of issues to consider. For some best practices, when it comes to creating art assets, please visit http://blogs.unity3d.com/2011/09/02/art-assets-best-practice-guide/.

Using Constructive Solid Geometry (CSG), commonly referred to as brushes, has been a long-existing way for games to have in-game level editors, which has led to people creating levels for them. Tools such as Unreal Engine 4, Hammer, Radiant, and other professional game engines make use of this building structure, making it quite easy for people to create and iterate through levels quickly through a process named whiteboxing, as it's very easy to make changes to simple shapes. However, just like learning a modeling software tool, there can be a higher barrier to entry in using brushes to create your geometry, but for creating those certain types of games where you need to create a lot of different content, it can be a great tool.

Unity does not support building things like this by default, but there are several tools in the Unity Asset Store that allow you to do something like this. For example, sixbyseven studio has an extension named ProBuilder, which can add the ability to create world geometry using brushes within Unity, making it very easy to build levels (it's what I use normally when building environments for 3D games in Unity and I use it in one of my other titles, Building an FPS Game with Unity, also available from Packt Publishing). However, as an extension, you'll need to buy it either from the developer or through the Asset Store, as it is not free. You can find out more information about ProBuilder and the free version of their tool at http://www.protoolsforunity3d.com/probuilder/.

Another way to generate architecture is through the use of tiles that are created by an artist. You can use them to build your level. Similar to using LEGO pieces, we can use these tiles to snap together walls and other objects to create a building. With creative uses of tiles, you can create a large amount of content with just a minimal amount of assets. This is probably the easiest way to create a level at the expense of not being able to create unique-looking buildings since you only have a few pieces to work with.