Old Castilian, or Old Spanish, is the early form of the Spanish language spoken from the 10th to the 15th centuries. It evolved from Vulgar Latin and was influenced by various regional dialects. Old Castilian is characterized by its distinct phonetics, vocabulary, and grammar, which differ significantly from modern Spanish. During the Middle Ages, Old Castilian became the literary language of the Kingdom of Castile. Notable works, such as El Cantar de los Cantares and La Celestina, showcase its rich literary tradition. Over time, it gradually transformed into the modern Spanish we know today, incorporating elements from other languages and dialects.