The largest fortress in Europe?

Here two answer options are suitable, depending on the strength criteria:
As an integral castle complex, the largest fortress can be considered Malbork in northeastern Poland. 300 kilometers from the capital, it was once the residence of the Teutons. Now it houses a museum.
Kremlin in Moscow. It is well preserved and is a museum and administrative center.
The Smolensk Kremlin, its walls are not solid, but the dimensions are larger than the Moscow Kremlin.
Walls of Constantinople. They contain about 100 towers, including 10 entrance towers, the longest in Europe. They are located in the European part of Istanbul.