Interesting read. Digging in and fortifying positions is very much still a crucial part of war. For example, in the current Russo-Ukrainian war, structures such as trenches, bunkers, basements, towers, buildings etc. are crucial for holding onto terrain. Also obstacle-like structures such as concertina wire, anti-tank ditches, dragon's teeth, minefields, czech-hedgehogs, etc. are all over the place. Wherever soldiers appear, they basically dig in and start fortifying, constructing structures both for their own protection as well as for obstructing enemy movement. An interesting recent development are kilometers-long anti-drone tunnels along key logistical routes, meant mostly for stopping rotary FPV drones that are trying to intercept logistics