Originally, Hadrian’s Wall ran across the whole width of Britain, and was about 10 ft thick and 15 ft high with 6 ft battlements on top of that. Every Roman mile there was a mile castle and in between each pair of mile castles were two watch towers.JPinoy wrote: Heck, ever heard of "Mile Castles"? Those small "residential fortifications" (not originally, but they eventually evolved into one) on Hadrians Wall? Those Castles existed long before the Middle Ages even began. With that in mind amongst others, one can say that "Castle Theme" can extend far back into the Ancients period.
If you see a castle as a fortification you are perfectly right.
As Sidney Toy says it in his book “Castles, their construction en history” so lovely:
“The art of fortification had reached a high state of development even at the dawn of history. Powerful military works, dating from the remotest periods, have been found in Asia Minor, in Greece, and in de basins of the Tigris, the Euphrates and the Nile rivers. The medieval castle, with its strategic, crenellated ramparts and aura of legend and romance, represents perhaps the most familiar form of a long-crucial art.”
So, fortifications were build from long ago till far in the 20th century. And therefore, it is not easy to draw a sharp line between a roman castellum, a medieval castle, and a “modern” fortification.