Here is visible a part of the ruined Long Walls of Athens.
The Long Walls in Ancient Greece, were walls built from a city to its port, providing a secure connection to the sea even during times of siege.
The phrase "long walls" generally refers to the walls connecting Athens to its ports at Piraeus and Phalerum. Those walls were constructed in the mid 5th century BC, destroyed by the Spartans in 404 BC after Athens' defeat in the Peloponnesian War, and rebuilt again with Persian support during the Corinthian War. They were a key element of Athenian strategy, since they provided the city with a constant link to the sea and prevented it from being besieged by land alone.
The original walls of Athens had been destroyed by the Persians during the occupations of Attica in 480 and 479 BC, part of the Greco-Persian Wars. After the Battle of Plataea, the Persian forces that had invaded Greece in 480 BC were safely removed, and the Athenians were free to reoccupy their land and begin rebuilding their city. Early in the process of rebuilding, construction was started on new walls around the city proper. This project drew opposition from the Spartans and their Peloponnesian allies, who had been alarmed by the recent increase in the power of Athens. Spartan envoys urged the Athenians not to go through with the construction, arguing that a walled Athens would be a useful base for an invading army, and that the defenses of the isthmus of Corinth would provide a sufficient shield against invaders; however, despite these concerns the envoys did not strongly protest and did in fact give advice to the builders. The Athenians disregarded the arguments, fully aware that leaving their city unwalled would place them utterly at the mercy of the Peloponnesians. y which he distracted and delayed the Spartans until the walls had been built up to such a height as to be defensible.
In the early 450s BC, fighting began between Athens and various Peloponnesian allies of Sparta, particularly Corinth and Aegina. In the midst of this fighting, Athens had begun construction of two more walls between 462 BC and 458 BC, one running from the city to the old port at Phalerum, the other to the newer port at Piraeus. In 457 BC, a Spartan army defeated an Athenian army at Tanagra while attempting to prevent the construction, but work on the walls continued, and they were completed soon after the battle. These new walls, the Long Walls, ensured that Athens would never be cut off from supplies as long as she controlled the sea.