Over several centuries, a series of castles and forts was constructed along the coast of the Solent to defend the harbours at Southampton and Portsmouth. These include the Roman Portchester Castle which overlooks Portsmouth Harbour, and a series of forts built by Henry VIII including Hurst Castle, situated on a sand spit at the mouth of the Solent, Calshot Castle on another spit at the mouth of Southampton Water, and Netley Castle. Southampton and Portsmouth remained important harbours when rivals, such as Poole and Bristol, declined, as they are amongst the few locations that combine shelter with deep water. "Mayflower" and "Speedwell" set sail for America from Southampton in 1620.