Awesome! I had been working on a similar project couldn't figure out the snow on the trees for the life of me! It's great to see how you approached it. The work look great! Thanks for sharing your process.
I believe author would be surprised that it's not american, but soviet bunker :)
William Vaughan has recently released his Pushing Points Topology Workbook which is a 125-page guide to the basics of SubD topology. The digital book comes with 60+ exercises, tons of tips and tricks, and useful techniques to help you get started.
There are over sixty exercises packed with loads of tips, tricks, and techniques designed to teach you how to manage the topology of your meshes.
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You can find more details on the book here.