Oftentimes, a combination of both. Archaeological evidence shows that an Alexandria is usually preceded by consistent inhabitation and commercial or agricultural activity, as is the case with the Alexandria of Egypt, or that an Alexandria has been super-imposed over an Achaemenid-era fortification. Although all of the primary sources regarding Alexander have been lost, an interesting case to examine is with the Alexandrias of Bactria and Sogdia, in modern-day northern Afghanistan and Tajikistan.

Two important ones are Alexandria-on-the-Oxus and Alexandria Eschate, both at the confluence of tributaries of the Amu Darya (Oxus River) and Syr Darya (Jaxartes River), respectively, and thus vital chokepoints for agriculture, commerce and the movement of the surrounding nomadic Iranian and Scythian peoples. Although Alexandria-on-the-Oxus (also known as Ai Khanoum) was developed greatly during the Seleucid and Graeco-Bactrian period, Alexander spent a disproportionate amount of time both capturing and fortifying the region around Alexandria Eschate; in the traditional account, he sieged and completely destroyed a city upriver of the proposed site he would fortify, called Cyropolis (an Achaemenid city marking the northeastern frontier of Iran). Next, as Sogdian tribesman resisted and fiercely fought the Hellenes, the Macedonians quickly erected a massive wall to fortify all of the Achaemenid fortresses they had captured, some 3.7 miles around the circumference of the new city.

The archaeological record confirms this: it seems that Alexandria Eschate was built from a preceding Achaemenid fortification, with remains from the 4th century BCE (when Alexander was conquering), though more excavation is yet to be done. As usual, the city was populated by wounded, infirm or elderly veterans of the campaign, whose descendants would form the elite of the city. In Bactria, there were a great many settlers and veterans -- a Seleucid history tells us that 23,000 settlers attempted to revolt in Bactria alone. Ai Khanoum, for example, though developed in the Seleucid era, has many palatial-like houses with Hellenistic artifacts, with maxims from the Oracle of Delphi, a temple containing a massive statue of Zeus, a gymnasium and a massive theater for the benefit of the large numbers of Greek elite in the area. What was more important to all of these cities, however, were the huge walls mentioned -- those of Alexandria Eschate are yet to be uncovered, but Ai Khanoum contains massive ramparts and citadel-towers, all for protection against the nomadic peoples outside the walls.

A lot of the Alexandrias would begin this way -- as fortifications, garrisons and citadels built by Alexander to hold down a vital area in danger of rebellion or nomadic incursion.

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