The long history of Jerusalem began well before it was captured by King David and made into the Capital of the People of Israel 3,000 years ago. Archeaological findings indicate the existence of a settlement in Jerusalem in the 3rd millenium BCE. The first mention of the city in historic sources begins in the 2nd millenium BCE.
The Ma'arot Writings, written in hieroglyphics, were meant to put a curse on the enemies of Egypt. They were written in the 18th and 19th centuries B.C., on small statues of prisoners or on bowls. The name "Rashlemum" (Jerusalem) is mentioned on some of them. The verse in Genesis 11;18 "and Malchi-Tzedek King of Salem brought forth bread and wine and he is priest to the Almighty God above," refers to that same period, which is known in the Bible as the period of the Patriarchs.
In the middle of the 2nd millenium B.C.E. the King of Egypt and his advisors carried on a volumous correspondence with the governors of the cities in the Land of Israel that were under Egyptian suzerainty. There was antagonism among these governors, and in their letters, pictured on the right, they complain about each other, and request help (one chariot or ten soldiers), to defeat their enemies, whom they describe, of course, as the enemies of the king. The letters were written in cuneiform, in the Akkadian language (which was the international language then, much as English is today), and some of them were found in Egypt, in the archive of the capital city, El-Amarna. Six of the letters found were written by the governor of Jerusalem ("Ershalem").
A Birds Eye View
The location of the ancient Canaanite city was chosen specifically for its natural protective qualities. The hill, on which early Jerusalem was built, has natural fortifications from three directions: the deep Kidron valley from the east, the "HaGai" (Tyropoeon) valley from the west, and the lowland where they meet in the south. The only side that isn't naturally protected is the north. This has been a problem that has accompanied ancient Jerusalem throughout its history, which has even been mentioned in biblical passages, such as the words of the prophet Jeremiah "...and from the north shall come the evil" (Jeremiah 1;14).
The Gihon Spring
In a land as dry as the Land of Israel, the main consideration in determining the location of a city or village, is its proximity to the nearest water source. The only permanent water source of ancient Jerusalem was the Gihon Spring. Its name is derived from the fact that it doesn't flow steadily, but rather in random eruptions with lapses in between them (Giha in Hebrew means eruption).
The City of David
The spring flows out of the ground from the foot of the hill, in the bed of the Kidron brook. The city was built on the top of the hill and on its slopes. This created a problem of access to water at times of war.
The city wall was built in the middle of the slope, which was the best location for purposes of defense. However, the spring remained outside the city defenses. In times of peace this fact was of little importance, but if the city was under siege, a serious threat of being cut off from the sole water supply arises.
During the 1960's the British archeologist Kathleen Kenyon excavated the eastern slope of the city's hill. She succeeded in exposing, at the middle of the slope, the remains of the solid Jebusite defense wall that King David had to overcome in his conquest of Jerusalem.
From the Biblical story of the capture of Jerusalem by King David, it is implied that the battle was won with the help of a stratagem connected with something called the "tsinor" (Samuel II, 5; 8). This word appears only here, its meaning is not fully known, and it has been translated as gutter or tunnel.
In the Jebusite city there was a method to access the Gihon spring water source, which is outside the wall, from within the city. A diagonal tunnel was hewn in the bedrock (apparently, along the line of a natural crack), and at its end a deep horizontal shaft was dug. From the top of the horizontal shaft, water jugs were lowered to the spring flowing below. Thus, access to the spring was hidden from the enemy outside the city. Perhaps this shaft is the "tsinor" through which King David's men climbed and penetrated the city as is mentioned in the Bible. The shaft was named after the British researcher Charles Warren who discovered it in the 19th century, . (Hezekiah's tunnel is from a later period).
And David dwelt in the stronghold, and called it the City of David. And David built a round about from Millo and inward." (Samuel II, 5:9).
After conquering the city, King David began its fortification. The wall on the east side of the city, which remained in the same place until the destruction of the Solomon's Temple, was built on top of the Jebusite wall on exactly the same course. Archeaolgical research has shown that was repaired many times over the years. The Millo (="fullness") is perhaps the filling that David's men had to pour on the steep slope in order to make it appropriate for building houses.
Jerusalem, the capital of the Jewish People for 3000 years, is located at the center of the Land of Israel, at the intersection of a number of ancient commerce routes. In Jerusalem, the North-South hilltop route intersects the main trade routes running from east to west.
Jerusalem was chosen by King David to be the capital mainly because the city, although part of the territory of the tribe of Benjamin, had not yet been conquered by the Israelites, and was not tied specifically to any of the twelve tribes.
For David, this was of great significance, because this enabled him to conquer the city with royal forces, and, as was customary at the time, retain it as royal property. He could use Jerusalem as the symbol for a united Israel. In order to emphasize the uniqueness and importance of Jerusalem, David brought the Holy Ark of the Covenant there and turned the city into the religious center of the People of Israel. He bought the threshing floor of Aravna the Jebusite and built an altar there to the Lord (Samuel II 24;21-25). Being a warrior, he was not permitted to build the Holy Temple himself. Therefore, he designated Solomon, his son and heir, to build the Temple after his passing.
Sources: Israel Ministry of Foreign Affairs