"The sacred area which included the temple, and the Athenian garrison were sheltered behind defensive walls with small towers built 66 feet apart. The fortification were constructed by the Athenians between 413-412 BCE, at the height of the Peloponnesian War when the Spartans with their allies were a constant threat to Attica, raiding both its surrounding land and interrupting its food supplies that came by sea. From the living area, a series of steps led to the sea where triremes were sheltered inside a double dock that was partially cut into the rock. The ship sheds were probably built in the 3d century BCE, and consisted of two roofed slipways, steeply rising out of the sea so triremes or other ships could be hoisted out of the water. Keeping triremes dry was of paramount importance because if their hauls were saturated with water they became heavy and slow."
— "Sounion", Ancient-Greece.org