The Persian Empire grew in the vacuum left by Assyria's destruction of the Kingdom of Elam (648 BC). Prince Teispes captured Anshan, once a stonghold of the Elamites. His father, Achaemenes is the person who is apparently responsible for training and organising the early Persian army and it is his name that is the beginning of the royal line of Achaemenian Kings. It is a dynasty which includes Darius the Great – the finest ruler of the Achaemenid era. The army he commanded included the infamous 'Immortals', who formed the elite of the Persian army, their numbers always kept to exactly 10,000 men.