On Thursday we caught an early train to Malbork, which is only about 30 minutes from Gdansk. It's the home of the largest brick castle in the world.
More a "fortress" than a "castle", Malbork Castle is half the size of the Vatican and was the home of the Grand Master of the Teutonic Knights, who had been charged by the Pope to civilize and bring God to the pagans living in (what is now) northern Poland.
The audio guided tour was 3.5 hours!!! Up staircases and across moats and through courtyards we went.
There were different living sections for different groups -- the Grand Master and his guests, the knights, and the monks. Each section had a large kitchen with a fireplace for cooking which was the size of my US bedroom! Each of the sections had its own dining room, sleeping areas, and area for relaxing.
There were stables housing hundreds of horses. There was a monastery and several chapels and churches. Central heating was achieved using metal tubes/grates leading from the furnaces to the floors above. The head chef and the Grand Master even had ensuite toilets!
We saw rooms full of chain mail, armor and weaponry, and we followed the progression of the weaponry as it became more sophisticated.
The castle was heavily damaged during the war, and has been reconstructed in its former style.
Fun tip! The knights were instructed in their rules to sleep in a large common room, on the floor, fully clothed, and the room should be always lighted. (Hmmmm)
The city itself is not much, so just stop in there, leave your bags at the train station, see the castle, and then continue on your journey. Five to 6 hours tops, including a nice Polish meal.