Battlements are actually a nice segway into something I would have like to add to the initial post above, but it didn’t quite fit the theme.
When people now talk about concrete, they mostly mean rocks or other aggregate fill bound with a water-activated cement. That’s what’s described above, and it works well enough. Go further in masonry, and you see things like stone blocks with the cracks filled with some sort of cement-based mortar. But what if you don’t have the materials or the knowledge to make cement-based mortar? This was apparently the challenge in post-Roman Sweden. In at least one intriguing instance, manufactured glass was used to construct the inner wall of a hillfort.
In the archaeology of the “Dark Ages”, vitrified fort is a term you see pop up from time to time. Scotland alone has nearly a hundred examples of stone strongholds with melted, glassy spots in the walls. Some of these are incidental- cooking fires built against the inside wall, or forge slag thrown in with the rubble fill. Some were clearly razed by a long-ago conqueror. Some may have been ceremonially destroyed by fire at the end of their useful occupation.
In a few instances, though, the glass-forming minerals poured over the fort’s rocky bones were imported to the site.So it was at Broborg, where the mineral amphibolite was melted on some sort of charcoal hearth and poured over the walls in long sections. This “deliberate and constructive act of man”  is still constructive in the present day: the 1500 year old walls of Broborg have something for us to learn when tackling the problem of stabilizing and disposing of nuclear waste in a way that will survive thousands of years into the future. 
Glass is a fun topic- I’m looking forward to tackling it in one of these journals!
P. Kresten “Swedish vitrified forts - a reconnaissance study,” Fornvännen 87 (1992) pp. 1-17 Link
J.L. Weaver, J.S. McCloy, J.V. Ryan, and A.A. Kruger “Ensuring longevity: Ancient glasses help predict durability of vitrified nuclear waste,” American Ceramic Society Bulletin, 95  (2016) pp. 18-23 Link