Long Walls
With the building of the Long Walls, Athens essentially became an island within the mainland, in that no strictly land based force could hope to capture it. (In ancient Greek warfare, it was all but impossible to take a walled city by any means other than starvation and surrender.) Thus, Athens could rely on her powerful fleet to keep her safe in any conflict with other cities on the Greek mainland. The walls were completed in the aftermath of the Athenian defeat at Tanagra, in which a Spartan army defeated the Athenians in the field but was unable to take the city because of the presence of the city walls; seeking to secure their city even against siege, the Athenians completed the long walls; and, hoping to prevent all invasions of Attica, they also seized Boeotia, which, as they already controlled Megara, put all approaches to Attica in friendly hands. For most of the First Peloponnesian War, Athens was indeed unassailable by land, but the loss of Megara and Boeotia at the end of that war forced the Athenians to turn back to the long walls as their source of defense.