I don’t know about the millennials, but I do know that those who were around when the movie came out in 1963 didn’t take to it, since it bombed at the box office.
It’s a decent action film, with some romance thrown in. Personally, I am more partial to Heston’s effort from three years later, as General Gordon in Khartoum. Though it is even more historically inaccurate than 55 Days, it was a thrill when I saw it as a kid in the theater. And I swear when I saw it they actually showed Heston’s severed head on a pole at the end, not the edited version that has been around for years.
After reading the comments on utube, is quite different from not being interested in it.
Anyhow, to each their own. I liked it!