There are two forts built over one another. The earlier vexilation fort dates to the period between the Conquest and the Boudican revolt. The other one is later. There has not been significant archaeological work to date the later fort.
http://www.pastscape.org.uk/hob.aspx?hob_id=66831
http://roman-britain.co.uk/places/rhyn_park.htm
https://historicengland.org.uk/listing/the-list/list-entry/1003716