[raketenjagdpanzer]

A lot of your material can come from demolished houses. Bulldoze up some buildings and you've got a workable rampart. Of course, speaking of buildings, fence between them and you now have a series of strongpoints connected by a defensive wall...

By example, part of my problem with the vision of Central Florida is that we're all basically suburbs here around a pretty porous city. Defending against a serious invasion here would be difficult. We have some natural barriers in places like swamps, but on the overall it's not a great area to try and defend. Mostly it's flat and covered with very convenient roads.

Trying to build a wall around Orlando, even one with a population of "only" 30000-50000 clustered near the middle of the city, would be a years-long undertaking requiring resources and people on an unprecedented scale, and huge I mean huge amounts of fuel for trucks to move material because honestly you're talking about millions upon millions of tons of earth, concrete and timbers. Even if optimistically you say 25% of your population can be part of the workforce and would be willing to be, without at least a few front end loaders and dump trucks working, circling an area like (again, for example) downtown orlando with a defensive wall of any consequence would be basically impossible to do in any practical length of time. Going by hand, by the time you got it finished you'd have the government back in control of the situation and it would be superfluous.

NOW WITH THAT SAID...

If you're talking about a small town, or a subdivision? Yeah, that's do-able! Heck, I look at the gated communities around here with their "ornamental" brick walls that surround them and access control points and think: yeah, that's DEFINITELY a good start! Some of them I see around here, you could top the walls with concertina and beef up the access gates and BAM - instant Keep!

Then, if you've got the fuel, you use your vehicles for "thunder runs" making sure each community is well supplied. Plus you could build up a comm network between them. Set, say, a mortar team locally to each neighborhood, and pre-plot some fire, then you train the local PD on how to call it in, give them a simplified map, overlap those mortar areas for mutual support...that's how I'd do it. Small walls, but a good support network.