By the time the Greek War of Independence broke out, in 1821, the Acropolis had become one of the most heavily fortified castles in Greece.
Three of its slopes were so steep that an attack there was impossible. The only accessible (therefore vulnerable) side was the western one, which led to its heavy fortification including two strong outer walls.
The first, known as the Wall of Hypapanti, protected the NW side of the hill. It was built in the 18th century to connect the castle (the Acropolis) with the city (Athens). The second, known as the Wall of Serpentze, was earlier. It had been built in the 17th century, on the ruins of two ancient monuments, the Odeon of Herodes Atticus and the Stoa of Eumenes to protect the SW side of the hill, as well as the vital wells on its southern slopes.In order to enter the castle of the Acropolis, one had to go through the gates of these two exterior walls, then 5 more gates of the bastions in front of the Propylaia, all the while passing in front of various defense positions along the way.
Bastions, walls and guns are but one of the many elements needed to effectively defend a fort. Another very important parameter is the access to supplies, of which water is the most vital.
The Acropolis had natural springs of water on both its southern and northern slopes (the spring of the Sanctuary of Asclepius on the former, the spring called Klepsydra on the latter). During the Middle Ages, cisterns had been built on the slopes as well as on the top of the hill.
Water supply was a decisive factor in the two sieges of the Acropolis during the Greek War of Independence. The first was during the first year of the fighting, when Greek forces besieged the hill, defended by the Ottoman Turks. The second was in 1826-27; this time the Greeks were besieged by Turkish forces intent on retaking the city.
In the first siege, the only source of water known to the defenders was, according to a testimony, a single well on the southern slope, within the Wall of Serpentze. The siege lasted from April 25th until July 20th, 1821, when Turkish reinforcements arrived to relieve the defenders. During this first phase, the main tactic of the Greeks was to concentrate their fire on the Wall of Serpentze in order to deprive the defenders of their only source of water. However, despite the artillery fire and an attempt to undermine the wall, the Greeks had not succeeded and were forced to break the siege upon the arrival of superior enemy forces.
They returned on November 4th 1821, when the second, longer phase of the siege began. On November 13th, the Greeks attempted a surprise night attack to take the Acropolis. They had passed the Wall of Seprentze and stealthily reached the gate just before the Propylaia when they were seen by the guards and came under fire. The offense failed but the Greeks maintained control of the Southern wall, including its precious well.
The Turkish defenders now had only what little water the cisterns could supply them. To make matters worse, during the winter it only rained twice in Athens, although its environs received the usual amount of rain. During the siege, Greek and foreign witnesses report that at the slightest sign of rain, the Turks would climb on rooftops and try to catch every raindrop by any means possible. A West-European traveler reports that they used sponges and even the women’s dresses to sponge off any moisture on the marble members strewn among the houses. Finally, the lack of water forced the defenders to surrender on June 10th, 1822, after 8 long months. Of the 2,500 defenders and non combatants, only 1,140 remained, most of them quite ill.
In the 4 years until the second siege of the Acropolis, the Greeks managed a few necessary repairs and made a few additions to the fortifications. The most important of these was a strong new bastion on the NW corner of the hill. They called it the Water Bastion, because they had discovered a spring of water there; it was none other than the ancient spring of Klepsydra.
Until the 19th century, the spring and the surrounding landscaping made in classical times had been buried underground. The only sign of a spring was a tiny rivulet flowing on the northern slope, mentioned by some European travelers, like Edward Dodwell.
The second siege of the Acropolis began in August 1826 and lasted until May 1827. This time the roles had been reversed: the Greeks were besieged by the Turks.
Knowing full well how important the Klepsydra spring was to the defenders, the Turks tried several times to destroy the Water Bastion, either by means of mines or by surprise attacks, all of which were successfully repelled by the Greeks.
The siege lasted 10 months, during which the Klepsydra, the wells of Seprentze and the cisterns provided enough water for the 2,250 defenders and non-combatants to survive. By the end of the siege, all the cisterns and wells had gone dry, except the Klepsydra. It is reported that its water was rationed to 640 grams (21.6 oz) per person, which had to suffice for drinking, bread making and cooking.
In the end, the Greeks surrendered, not for lack of water, but because they had run out of food and medical supplies, while the Greek army had failed to relieve them.
Despite the heavy Turkish fire, the diseases and the lack of food, 2.110 of the defenders survived.
This and many other examples of military history prove beyond doubt that preparation and logistics are vital for the successful outcome of a battle.
 Τανούλας, Τ., Τα Προπύλαια της Αθηναϊκής Ακρόπολης κατά τον Μεσαίωνα, Αθήνα, 1997.
 Σιμόπουλος, K. Πώς είδαν οι ξένοι την Ελλάδα του ’21, 1984, σελ. 467, σημ. 42.