Complex sentence: That year, the Spaniards built the presidio of San Marcos de Apalache on Apalachee Bay, and in 1721 the British built Fort King George at the mouth of the Altamaha River.
Split into simpler sentences: That year, the Spaniards built the presidio of San Marcos de Apalache on Apalachee Bay. In 1721 the British built Fort King George at the mouth of the Altamaha River.