The documentary does a fairly good job of explaining the brochs of Iron Age Scotland, basically large conical stone towers of the northern British Isles. They exist as far north as Shetland Islands in the subarctic archipelago. The evidence for brochs going back to the 4th century BC is a bit tenuous, which the video doesn't really explain in depth, but many of them can be reliably dated to the 1st centuries BC and AD despite the paltry evidence of radiocarbon dating. That would make them contemporaneous with the first Roman landing in the British Isles by Julius Caesar in 55 and 54 BC, and later with the Roman invasion of 43 AD, the revolt of Boudica in 60-61 AD, and the Battle of Mons Graupius in 83 AD when the Roman army under Gnaeus Julius Agricola invaded Scotland.
The Romans, who built the baths of Bath, England, Hadrian's Wall, and urban sites such as Londonium (modern London, where a Roman wall section still exists), obviously brought a grand Greco-Roman and Mediterranean architectural tradition to the British Isles, one that would help shape the architectural traditions of later cultures such as the Anglo-Saxons and Normans. However, the pre-Roman Britons, Cornovii, Caledonians, Caereni, and other Celtic natives clearly had one of their own in the far north, and not just something inspired by Celtic oppida fortress towns in mainland Europe. As the video explains, these towers don't exist further south in England or Wales.
Can anyone share their thoughts, theories, or insights on how the brochs developed and evolved within the local culture? We still don't know their exact purpose, if they were used solely for defense as strategic fortifications, or if they were stately manner homes of local lords, or a combination of both. Some are placed in strange areas, though, that don't seem to make much strategic sense, and could have even been used as religious sanctuaries (although nothing like that has been proven, of course).