You will need to first clarify which Alexandria you mean when you refer to the "original city" because that will impact the answer. If you mean where was the first city named Alexandria, that is a different answer than asking where the earliest settlement for the most famous Alexandria was located. There were many Alexandria's that were founded and the location of the most famous city, the capital of the Ptolemaic dynasty and the location of the Pharos Lighthouse and the Museaum and Library, is not a mystery. It is around and beneath the location of modern Alexandria, on the shores of Lake Mareotis and the Mediterranean, near the Nile Delta.