What does "roundel" mean?

Roundel: Anything having a round form; a round figure; a circle. The Spaniards, casting themselves into roundels, . . . made a flying march to Calais. Bacon. Specifically: (a) A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries. (b) (Her.) A circular spot; a sharge in the form of a small circle. (c) (Fort.) A bastion of a circular form.