The Ptolemaic fortified base of Arsinoe in the Peloponnese is located on the eastern side of the Methana peninsula, facing the island of Poros. The base was located on the Nissaki, joined to the peninsula by a narrow spit. Beyond it, and to the south, was an inlet that contained (according to an inscription relating to a boundary dispute) Ptolemaic naval installations, a drag way, as well as tunny traps. This was adjacent to the narrow isthmus that joins the peninsula to the Troezenia.
For further details about the base see here (“Arsinoe in the Peloponnese: the Ptolemaic base on the Methana peninsula”).