Glad to see that you too will be joining our cruise. I was in dover a year ago. The castle is with in sight of the ship. However being a castle its on a hill or rather the cliffs. Its an easy taxie ride from the ship that shouldnt cost you any more than 5 pounds.
In my five weeks spent touring the english country side, Dover Castle and the tunnels were probably one of the most impressive things a saw. I plan to take my family up to the castle when we arrive in dover. Granted we will be jet laged but it will be well worth it.