A bit far from historic city center (around 40-45m walking) but is pretty reasonable just for one or two day stops because you are near Central Train Station (a bit less than 20m walk) and you will probably only go to the historic center once.
For me was perfect because I just needed to catch a train to Amsterdam, so I could enjoy a nice day on Antwerp and was near my train next morning.
Leonardo is really friendly and rooms are nice, so I recommend his place for short stays or if you need to do some other stuff besides tourism.