I want to have Washington, DC have a militarized defensive wall (called the Federal Wall) somewhat similar to the Hoover Dam but with artillery, air, and infantry support stationed on its (imagine something like this from Fallout (https://vignette.wikia.nocookie.net/fallout/images/d/db/FNV_concept_art_Hoover_Dam.jpg/revision/latest?cb=20100504192604 (https://images.app.goo.gl/4gApVLP3QVW5N45Y7))). My original idea was to have the Federal Wall basically take up I-495 (https://hobnobblog.com/wp-content/uploads/2013/06/beltwaymap.gif (https://images.app.goo.gl/6B66h5MHvisd24o6A)) in order to make Washington itself a very well defended area but after some thinking, I realized that making the wall go around I-495 was impractical and would be a poor us of resources and manpower. So instead, I’ve decided, I’ve decided to have the Wall protect and encircle Capitol Hill, the Southwest Federal center, the entirety of the National Mall, most of Foggy bottom, West End, DuPont Circle, U Street Corridor, Shaw, Truston Circle, NoMa, and Near Northeast (basically the center of Washington with all of the important buildings). The wall itself won’t be anything super crazy like the three walls from Attack on Titan. Imagine a slightly bigger/wider Great Wall of China that’s able to service aircraft, howitzers and mortars, trucks/support vehicles, infantry, etc.
Although the District of Columbia has greatly expanded its territory to basically directly control the entire DC metro area as well as large swaths of northern Virginia and bits of southern Maryland, the Federal Wall will be used to still act as a last ditch defensive structure in order to defend against both potential marauders and bandits as well as near-peer enemy militaries.
But in a modern (or rather, post-apocalyptic) context, how well do walls work? I know that walls are considered impractical in today’s day and age since modern military hardware and technology like aircraft and artillery can bring down any wall but would they still make sense as a physical protective barrier in a post-apocalyptic setting?