 By the time Elizabeth I came to the throne in 1558, most of the royal fleet used this section of the Medway, known as Chatham and Gillingham Reaches, as an anchorage.Although the Thames had been defended from naval attack since Henry VIII's time, when five blockhouses were built as part of the Device Forts chain of coastal defences, there were no equivalents on the Medway.Two medieval castles – Rochester Castle and Queenborough Castle – existed along the river's south bank, but both were intended to defend landward approaches and were of little use for defence.