The geography of ancient Greece shaped its development. Greece is a peninsula surrounded by water on three sides, with a narrow isthmus connecting the Peloponnesus peninsula. Most of Greece's landscape is mountainous, making travel difficult and hindering political unification. The sea influenced Greek civilization greatly. Most Greek cities were built on harbors to facilitate trade via the Mediterranean, Ionian, and Aegean Seas. Trade helped Greece prosper and exposed Greeks to new ideas from other cultures.