With no further threat by the Serbs and the subsequent Byzantine civil wars, the Ottomans besieged and took Constantinople in 1453 and then advanced southwards into Greece, capturing Athens in 1458. The Greeks held out in the Peloponnese until 1460, and the Venetians and Genoese clung to some of the islands, but by the early 16th century all of mainland Greece and most of the Aegean islands were in Ottoman hands, excluding several port cities still held by the Venetians (Nafplio, Monemvasia, Parga and Methone the most important of them). The mountains of Greece were largely untouched, and were a refuge for Greeks who desired to flee Ottoman rule and engage in guerrilla warfare.