 The Solent , a stretch of water that gave access to the ports of Southampton and Portsmouth , was considered vulnerable to attack . Two castles were built on the west and east sides of the River Medina , the entrance to the principal harbour of the adjacent Isle of Wight , and were intended to attack any enemy ships that might approach . The castles took their names from the traditional term for this location , " the Cowes " . Constructed in 1539 , the western fortification , which became known as West Cowes Castle , had a D @-@ shaped bastion at the front , a round , two @-@ storey keep at the rear and two low buildings on either side , all of which mounted artillery guns . The landward side was protected by a ditch 4 metres ( 13 ft ) wide and a stone wall at least 2 @.@ 4 metres ( 7 ft 10 in ) wide at the top . It was built from limestone ashlar stone , which may have been recycled from the demolition of nearby Beaulieu Abbey , a consequence of Henry 's recent dissolution of the monasteries . 
