The Matanzas River, an arm of the ocean beginning just south at Matanzas Inlet, was an important water approach to the Spanish town of St. Augustine, 13 miles to the north, from the earliest colonial days after Spain established the town in 1565. As early as 1569, wooden watchtowers were built in the vicinity to watch the river approach to the town. In 1740, the Spanish began construction of Fort Matanzas, a unique fortified watchtower built from coquina, a local sedimentary stone composed mainly of mollusk shells. For eighty years, Fort Matanzas guarded St. Augustine's back door and remains the last remnant of the town's colonial outer defenses.