Something there is that doesn’t love a wall.
People sometimes say of poor deluded Donald Trump that he says what he feels rather than what he thinks.
He certainly hasn’t been thinking about this ‘wall’ idea of which we hear so much. It’s a lousy idea (for those who have been attending to more important things, US presidential candidate Trump suggested dealing with the infamous porosity of the US-Mexican border by building a wall along its length).
Of course he is thinking of Hadrian’s Wall or the Great Wall of China. Both of these were built wherever possible along natural, geologic formations. Also the people they were supposed to exclude (or in the Chinese case probably also include) weren’t likely to be packing a siege train.
Were Trump more of a scholar he might have heard of the Limes Germanicus, a 350-mile barrier built by the Romans to exclude the rude and boisterous Germans from decent Roman society. The Mexican border is more like 2000 miles, eight times the size, but then President Trump would be at least eight times the size of the emperor Hadrian.
Though I can find no equivalent online there is a fine line drawing of a section of the Limes in Kaufmann & Kaufmann: The medieval fortress [ISBN-13: 978-0306813580].
The modern equivalent would doubtless resemble sections of the Inner German Border during the Cold War.
Of course without some kind of naval precautions around the coastal ends of such a structure it wouldn’t be worth building. I don’t think he’s thought about that either.