City walls and fortifications were of extreme importance to cities within the Archaic and Classical Periods. Before the introduction of siege artillery in the 4th Century sieges heavily favored the defense, and assaulting walls directly was extraordinarily difficult and costly. Beginning in the Archaic Period the settlements on the Greek mainland started to fortify themselves with walls, and in many colonies (Old Smyrna, where most of the population was huddled close together behind the walls, being the classic example) the walls or the citadel were the first things set up. This period of fortification coincides with the increasing establishment of πολεῖς, and city walls were heavily associated with a city's political freedom (αὐτονομία). The Thebans, for example, tore down Thespiae's walls after Delium in an effort to finally destroy a rather stubborn (and decidedly pro-Athenian--a pro-Athenian insurrection broke out in the city in 415) member of the Boeotian League's political liberty. Sparta's lack of walls (until the Hellenistic Period, although there may have been a small inner-city wall as early as the Peloponnesian War) was notable not simply because of the misplaced confidence that they could repel any military attacker without one (which the Thebans showed was nonsense--all one had to do was isolate the city from the subjects on which it relied and it would be unable to maintain itself) but because it was yet another indication that Sparta had never really become a πόλις in the same way that most cities on the mainland had--along with her lack of walls, Sparta maintained an archaic dual kingship and even in the Augustan Period was still composed of several villages (Pitana, Mesoa, Limnae, and Cynosura) rather than a united city, a political organization more fitting for a Dark Age or Archaic site (c.f. Archaic Corinth). It's not much of an exaggeration to say that in the minds of Classical Period mainlanders without walls there was no city, at least not as a politically independent entity, especially after the Persian invasions, when even minor cities began constructing defensive walls.

Athens was no different. Her first defensive structure was the Acropolis, which was fortified with a wall during the late Mycenaean Period, but this was little more than a citadel enclosing the handful of Mycenaean sites within. Her first important fortifications were built during the Archaic Period and consisted of a wall enclosing the city. Little is known about it, however, because of course the wall was destroyed during the Persian sack of the city before Salamis, along with the rest of the city. When the Persians withdrew from the city it was left in shambles, with most of the houses and buildings torn down and scattered as rubble all over. Much of this rubble was collected and either dumped down wells (by virtue of which an enormous amount of late-Archaic and early-Classical pottery has survived) or was buried under the Acropolis in what's called the Perserschutt, where a huge body of Archaic and early-Classical statuary has been discovered (including my personal favorite Archaic statues, the Korai of the Acropolis). Much of the rubble, however, found itself incorporated into the hastily-rebuilt city walls, which was pretty much literally the first thing that the Athenians did after re-occupying the city--famously, according to Thucydides, the Spartans tried to prevent the Athenians from rebuilding their walls (to prevent them from regaining their political strength) and Themistocles successfully stalled for time long enough for the walls to get slapped together. With the construction of the walls the autonomy of the city was preserved

Athens' most famous fortifications, though, were the Long Walls, which were copied at Megara, Argos, and even oligarchic Corinth. Suggested by Themistocles but not actually begun until 461, the Long Walls (often called the "Legs," σκέλη, as were the Long Walls at Megara, a pun that Aristophanes could not resist in the Lysistrata) were a set of parallel walls extending from Athens to her two seaports, Phalerum and Piraeus--around 6 km. These walls were more or less impregnable, even more so after Pericles built a third wall, securing the causeway to Piraeus once and for all, and turned Athens and Piraeus into a fortress big enough for the entire population of Attica to live in during times of war, and thus freeing Athens, whose population was vastly inferior to the troops that the Peloponnesians and their allies could raise, from the need to challenge an invader in open battle within Attica. Safe from behind the walls the population of Attica could allow an invader to waste his time in Attica (as the Spartans did during their fruitless invasions during the early part of the Peloponnesian War), while the Athenian fleet supplied the city and engaged in offensive raids along the enemy's coast. As a grandiose display of power it was a symbol of the Athenian Empire's strength, and of the political importance of the city within Attica (which would cause tension later, as rural inhabitants were not too thrilled by the idea of retreating for months or even years at a time while their homes were left to the invader). As an acknowledgement of the importance of the fleet it was a symbol of the democracy itself. Politically the fleet was everything to Athens, since it was the bastion of the democracy--because of the need for an enormous body of rowers, the maintenance of the Athenian fleet meant that in essence every citizen was expected to provide military service to the state, which in turn meant that according to the logic of the constitutions of most Greek states, which linked military service to the right to political power, every citizen at a fundamental level had the right to expect to exercise power in the Assembly. Thus the democracy was very tangibly and practically grounded, and the Long Walls were an acknowledgement of that. The symbolism was not lost on the Athenians--the Long Walls are constantly equated with the democracy itself, and Thucydides even remarks that it was the enemies of the democracy that attempted to stop the construction of the walls. And the Long Walls were not just recognized as a symbol of Athenian power and democracy by the Athenians themselves--the Spartans demanded that the Long Walls be torn down after the Athenian surrender in 404, and their demolition was accompanied by rejoicing and troops of flute-girls. Athens rebuilt them at the first chance possible, in 393 under Conon during the Corinthian War, which coincides with the revival of the democracy and the fleet as well as the beginnings of the Second Athenian Empire shortly after the Corinthian War. Though they fell into disrepair during the Hellenistic Period they were still the linchpin of Athens' defense as late as 86, when Sulla breached them and isolated both Athens and Piraeus.