Round sumo ring

...From the end of the 17th century, sumo rings were lined up on the ground in a square shape, and matches were decided within these boundaries. During the Genroku period (1688-1704), this was standardized to a circular ring, and eventually rice bales were divided into thirds to make thinner bales, which were buried about six percent in the ground. The ring area, which had previously been flat, was built on a high platform of piled-up soil, and outer bales were buried on all four sides of the outer edge to prevent soil buildup, making it easier for spectators to see. The appearance of the ring and the changes in sumo ring construction thus gave rise to the use of techniques such as pushing the opponent outside the ring or pushing him out by pulling him out, in addition to matches that had previously been decided by knocking him down inside the ring. ...

*Some of the terminology that refers to the "en dohyo" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…17世紀末ころから相撲場に，土をつめた五斗俵を四角に巡らして地面の上に並べておき，この境界線の中で勝負を決めるようになった。それが元禄時代(1688‐1704)に円土俵に統一され，やがて米俵を3分して細俵をつくり，土中に6分くらい埋め，それまで平地にあった土俵場を土を盛った高い壇上に築き，外側の縁にも外俵を四方に埋めて土止めとし，見物人に見やすいようにした。このように土俵の出現と相撲場構築の変化とは，それまでほとんど円土俵内で相手を倒して決めていた勝負のほかに，円土俵の外へ押し出したり，寄り切ったりする決り手を生んだ。…

※「円土俵」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報