What is "rondels"

Usage examples of "rondels".

Ignoring the path to the side, he mounted broad steps, crossed a verandah and approached a pair of narrow twelve-foot-high doors, studded with iron rondels bearing flying snake motifs.

Triolets, villanelles, rondels, rondeaus, Seeds in a dry pod, tick, tick, tick, Tick, tick, tick, what little iambics, While Homer and Whitman roared in the pines?