The Abbey of Santa Maria a Mare dominates the island of San Nicola from its clifftop position, its fortified walls giving it the look of a castle as much as a monastery. The Benedictine foundation dates to the 9th century, though the complex as it stands today is largely 11th–15th century. The church contains a remarkable 11th-century mosaic floor — one of the best-preserved in southern Italy — and a Byzantine crucifix of considerable age and artistry.
The abbey is the principal historical monument of the Tremiti Islands and the main reason to spend time on San Nicola rather than simply using the ferry as a shuttle to San Domino's beaches. The fortified gateway, the cloister, and the various medieval buildings clustered within the walls reward slow exploration. A small museum within the complex holds artifacts recovered from the islands and the surrounding seabed, including Roman-era material from the ancient settlement. The views from the abbey walls over the Adriatic are expansive.