How long is the Israeli wall, how long is the Egypt/Gaza Wall, and how long would the US/Mexico wall have to be? Also, how long is the new Hungarian wall? The US wall will have to be close to 2,000 miles. Each of those miles will have to have a number of people guarding the wall to prevent breaking through, climbing over, tunneling under, etc. Also would be needed a staging facility with food, bathroom, office, bed, vehicles, etc. to maintain 3 shifts of personnel for each mile. How closely would the guards need to be placed, how close would the staging facilities need to be etc. Anyone want to estimate the maintenance cost of all that, plus the building of both the wall, the roads, the facilities, the training of the guard personnel, etc.?
So far as the Great Wall off China is concerned, there were actually several walls, built over periods of centuries, and they didn’t always keep the invaders out. They were more important as a paved road to travel quickly over rough terrain to quickly pass messages and get troops to where they might be needed for invasion in force. Here are hundreds of images of the walls with towers in high places, probably where troops and guards were stationed, and signals sent. Note that the crenellated walls along the road vary in places. Some have embrasures on both sides for archers, others only have them on one side, presumably facing the enemy. The mountains are impressive and you can see why roads were less popular.
At various times China has claimed different mileages of wall. The link below discusses potential claims and possible use for land disputes with Korea.
In all, when it came to major invasion, the Chinese wall did not keep the barbarians out. Unless they had large numbers of troops available to mass at an attack point, it was not that hard to get over the wall.
True to a certain extent but they didn’t have the other technologies that we have as well and realistically simple things like minefields might eventually be necessary. The invasion has been going on and has to be stopped one way or another or we can kiss the country goodbye and start speaking Spanish and bribing cops and government officials.
The wall will be a physical barrier, and also use electronic monitors to detect approaches, tampering, etc. Bases the grateful locals will welcome will be set back from the border and feature drones and helicopters. The Great Wall, by contrast, wasn’t built all at once; it took its current form sometime in the Middle Ages; Marco Polo’s Travels (which was ghostwritten) doesn’t mention it; and we won’t be using rice flour and mud to build our wall.