The Muiderslot is a castle that was built in the 13th century as a defensive building, but was never used as such.
Nowadays, it is a museum, and many school classes in Holland have paid a visit.
Although the Muiderslot is in Muiden - not in Amsterdam, the castle is recently marketed as Amsterdam Castle by the tourist board. This has apparently greatly increased the visitor numbers.
The castle is located in Muiden, which is about 14km/9mi from Amsterdam. One of the ways to get there is by ferry boat from IJburg, but it also makes a nice bike trip.