Why was aberdeen city center chosen as a settlement?

In the middle ages, Aberdeen was settled on the banks of the River Don. This part of the city is now known as Old Aberdeen and still has its own cathedral and university dating from 1494. As the town expanded, it moved southwards towards the River Dee which could be used for shipping and fishing purposes. The Central area soon became developed as the town grew and now straddles both banks of the rivers Dee and Don and all the area in between. The main street, Union Street was laid out in the 18th Century and Aberdeen is now the main centre for the UK's North Sea oil industry. The population is in the region of 300,000.