土 (earth) is the indexing radical. A 域 is a territory, region, or domain - a stretch of land marked off and held. The earth radical groups it with 城 (city), 境 (boundary), 圈 (zone) and 界 (border) in the geographic vocabulary.
或 (huò) supplies the sound, drifted to yù. Historically 或 itself pictured a halberd guarding a square - the original ancestor of both 域 and 国 (country, also enclosed). So the phonetic still carries an echo of bounded territory. Same series: 域, 惑, 蜮.
