Oude Haven, the old harbour was created around 1350 as a result of the damming of the river Rotte. As the name might suggest it is the oldest harbour in Rotterdam. Moored here are many historic ships which have been carefully renovated. There are other artefacts from the past including a train.
Above is one of the various boats found in this tranquil area of the city. The harbour is a real contrast to the enormous modern harbour structures.
This wonderful lightship Vessel 11 has been carefully restored and people are able to eat and drink on it.

Also check out the article on Delfshaven which was developing at the same time
References
https://www.cityguiderotterdam.com/things-to-do/sights/old-harbour-rotterdam/