^ Where is it? near London or at Malta?
^ Here it is said that Polygonal Forts existed since Napoleonic days. all before rifled artillery of any kind were invented (implying that poly forts were made to response the threats of shell firing howitzer proliferations). Wikipedia entry said it 'comes BOTH with proliferations of shell guns (Howitzers to be specific) AND Rifled artillery. When exactly did the first Polygonal Forts were designed or built.
Also i'm not sure if casemates were 18th Century inventions, AFAIK a casemate might be as old as the first siege cannons first evolved from Big Bombards.
And sadly no games ever presented polygonal fort as any historical significant things while IRL British Empire was very much serious about it. as the proliferations of howitzers (and also at the time of Palmerston, rifled artillery) rendered oldschool starforts obsolete. so permanent fortifications needed to evolve oncemore... this for isn't even presented in Civ6. this might be because there were originally Urbaization Civic, and later Steel technology that gave free city defenses to all player cities and thos no forts are buildable from that point on.