What does marcellus mean when he describes the humans as being “glued to their horses”

When he says the Huns were "glued to their horses" he is saying that they were such good and well-trained warriors that it seemed impossible to knock them off with weapons.

the answer should be arrogance & pride.

frederick william of brandenburg-prussia trained his son, frederick, to be a hardy soldier. but frederick disliked rifle shooting and horseback riding. unlike his father, he was interested in french literature. he enjoyed poetry and music and, like some others of the enlightenment, he scoffed at religion. his father thought him soft, frivolous and unmindful of his duties. and when frederick refused to put his signature on a right to succession, for a few moments his father strangled him.