The biggest of the old castle built by the Teutonic Order in Poland is the Malbork castle. It is well worth a visit. On the castle is a large museum of amber which is included in the admission ticket to the castle.
Amber has traditionally been an important trading commodity for the countries at the shores of the Baltic Sea and a large amber trade in this area has been going on for more than two millennia.
The museum gives a good introduction to the history of amber in Poland and it is well worth heading into the museum so you can get your amber fix here instead of having to go to another museum somewhere else.
There are a lot of different types on amber on display so you can see what kind there is out there. One of the most interesting displays is the section with the many pieces of art which has been made out of amber. Some of them are quite big and must have taken a lot of effort to produce.