Although the oasis is known to have been settled since at least the 10th millennium BC, the earliest evidence of connection with ancient Egypt is the 26th Dynasty, when a necropolis was established. During the Ptolemaid period of Egypt its ancient Egyptian name was sḫ.t-ỉm3w, "Field of Trees". Greek settlers at Cyrene made contact with the oasis around the same time (7th century BC), and the oracle temple of Amun (Greek: Zeus Ammon), who, Herodotus was told, took the image here of a ram. Herodotus knew of a "fountain of the Sun" that ran coldest in the noontide heat. During his campaign to conquer the [Persian Empire], Alexander the Great reached the oasis, supposedly by following birds across the desert. The oracle, Alexander's court historians alleged, confirmed him as both a divine personage and the legitimate Pharaoh of Egypt, though Alexander's motives in making the excursion, following his founding of Alexandria, remain to some extent inscrutable and contested. The Romans later used Siwa as a place of banishment. Evidence of Christianity at Siwa is uncertain, but in 708 the Siwans resisted an Islamic army, and probably did not convert until the 12th century. A local manuscript mentions only seven families totaling 40 men living at the oasis in 1203.
The first European to visit since Roman times was the English traveler William George Browne, who came in 1792 to see the ancient temple of the oracle.
The oasis was officially added to Egypt by Muhammad Ali of Egypt in 1819, but his rule was tenuous and marked by several revolts.
Siwa was the site of some fighting during World War I and World War II. The British Army's Long Range Desert Group was based here, but Rommel's Afrika Korps also took possession three times. German soldiers went skinny dipping in the lake of the oracle, contrary to local customs which prohibit public nudity.
The ancient fortress of Siwa, built on natural rock (an inselberg), made of salt, mud-brick and