How does elizabeth assert her royal authority in this speech? how did elizabeth's choice of clothing underscore her declarations?

She starts by reducing the distance between her royal persona and her soldiers by means of starting with a statement of the reciprocal love that reigns (pun intended) between her and her people. Because of such love, this relationship is a relationship of trust, courage and proximity as she declares her intentions of dying with her soldiers if need be. She then uses a literary device called conceptual chiasmus, in which the orator makes a statement in a first sentence and then reverses it in the second sentence, when she says:

"I know I have the body of a weak, feeble woman; but I have the heart and stomach of a king"

By saying that she is accomplishing a twofold purpose, she is not only asserting her superior strength as an English monarch over Spaniards, she is also asserting her royal authority over her subjects and her refusal to be predetermined in terms of moral and emotional strength by her gender.

Her choice of clothing has the exact same purpose and effect. She is dressed in a white velvet gown, covered by a cuirass and her head covered by a metal helmet. The clothing choice stresses her perfectly balanced duality as the velvet gown represents her feminine part and her cuirass her masculine part. Furthermore, as an educated Renaissance noblewoman her appearance is reminiscent of Greek mythology as she resembles Pallas Athena, the Greek goddess of war.