Ah, the River Adur, which was navigable up to Steyning in Norman times. That has silted up over time, which is true for many of the old ports. Also, the size of ships in Norman times was much smaller: by 1400, they were increasing in both draught and length, with a result that many ports ceased to be suitable for them (Lostwithiel on the Fowey is an example - it was a thriving port in 1200 and the administrative centre for Cornwall; by 1450, ships could no longer reach it). As ships ceased to use rivers, they silted up as nothing passed to stir up the sediment. In the last 100 years, the upper reaches of the Tamar (from Calstock onwards) have suffered from this. I can't find any references to suggest sea levels have altered much since 1066.<quoted text>The castle was Bramber. The river was tidal and just west of Brighton. The amount of water is constant. It may be ice, vapor or liquid. The only increase in sea level would be ice that is over land melting and running into the sea
Of course you are right about the amount of water - however, its conversion from ice to water is the worry as this has the impact of raising sea levels.