What language did Korea use before Hangul?

Gugyeol was developed during the Unified Silla period (668 - 935 CE) and continued to be used throughout the Goryeo Dynasty (918 - 1392 CE). It used modified Chinese characters combined with unique markers to indicate Korean vocabulary and grammar. Although Gugyeol simplified certain aspects of writing, it still relied on understanding Chinese characters, making it difficult for the general population.