Not all English Heritage properties are open yet, and those that are require pre-booking via their website.
If you, or any of your party have a new persistant cough, or develop a fever of 38C, we would instead advise you to use the NHS's dedicated website for professional advice.
One of the best examples of Norman military architecture still standing, set in riverside gardens. Begun in AD1087, the 4-storeyed keep, one of the tallest in England at 113ft high, was added from 1126-39 and is partly built on the Roman city wall and offers excellent views. The rest of the castle was demolished in 1610, leaving just the keep and the curtain walls. There is also a model of the castle, showing how it might have looked in the 14thC.