Portchester Castle, Hampshire
It's not often you find a medieval castle built into the walls of a 3rd century Roman fort but Portchester Castle near Portsmouth, is a fine example. Unfortunately, renovation works rather ruined the façade but once inside the ruins are a joy to explore.
Managed by English Heritage (it's worth being a member) and owned by Southwick Estate, this Grade I listed building and Scheduled Ancient Monument even has its own Norman church in the grounds of the fort.
Over the centuries the castle has seen many tenants, notably King John, Edward II and Henry V. Research further into its history and you will find a very interesting and name studded past indeed. Look for the time-worn graffiti as you ascend the tower and the art left by prisoners when the castle served as a gaol.
It's free to enter the grounds of the fort.