Matthias Noback has a series of posts on his site sharing some of his thoughts around layers, ports and adapters in application architecture:

Looking back at my old blog posts, I think it's good to write down a more balanced view on application architecture than the one that speaks from some of the older posts from 2013 and 2014. Before I do, I allow myself a quick self-centered trip down memory lane.

He's posted all three parts of the series, each providing either some background on him and his previous work or principles and architecture suggestions:

• Layers, ports & adapters - Part 1, Foreword
• Layers, ports & adapters - Part 2, Layers
• Layers, ports & adapters - Part 3, Ports & Adapters

Each part includes code and/or structure suggestions to help clarify points made along the way.