In the Middle Ages, the ruins of old Roman forts were often built on top of to create medieval castles. One great example of this is the so-called Multangular Tower in York, constructed when the city was called Eboracum by the Romans and visited by emperors as distinguished as Septimius Severus and Constantine. The latter was particularly known in York as his father had been governor of the province.
If you look at the photos I took on my visit below – you’ll notice that the bottom half of the tower has smaller stones and lines of red bricks one associates with Roman forts. Then above you have the addition of arrow slits from the medieval period. The walls would probably have fallen into a certain degree of decay in the centuries immediately after the Romans left but wars with Scotland in particular, meant that the medieval city had to re-fortify later on.